doc: use US English for consistency

behaviour -> behavior

PR-URL: #2784
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Brendan Ashworth <brendan.ashworth@me.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>

CONTRIBUTING.md

@@ -198,7 +198,7 @@ CoC](http://www.rust-lang.org/conduct.html).
 * Please keep unstructured critique to a minimum. If you have solid
   ideas you want to experiment with, make a fork and see how it works.
 * We will exclude you from interaction if you insult, demean or harass
-  anyone.  That is not welcome behaviour. We interpret the term
+  anyone.  That is not welcome behavior. We interpret the term
   "harassment" as including the definition in the [Citizen Code of
   Conduct](http://citizencodeofconduct.org/); if you have any lack of
   clarity about what might be included in that concept, please read
@@ -212,7 +212,7 @@ CoC](http://www.rust-lang.org/conduct.html).
   a newcomer, we care about making this community a safe place for you
   and we've got your back.
 * Likewise any spamming, trolling, flaming, baiting or other
-  attention-stealing behaviour is not welcome.
+  attention-stealing behavior is not welcome.
 * Avoid the use of personal pronouns in code comments or
   documentation. There is no need to address persons when explaining
   code (e.g. "When the developer")

doc/api/modules.markdown

@@ -301,7 +301,7 @@ The `exports` variable that is available within a module starts as a reference
 to `module.exports`. As with any variable, if you assign a new value to it, it
 is no longer bound to the previous value.
 
-To illustrate the behaviour, imagine this hypothetical implementation of
+To illustrate the behavior, imagine this hypothetical implementation of
 `require()`:
 
     function require(...) {

doc/api/process.markdown

@@ -206,15 +206,15 @@ Note:
   install a listener but that won't stop the debugger from starting.
 - `SIGTERM` and `SIGINT` have default handlers on non-Windows platforms that resets
   the terminal mode before exiting with code `128 + signal number`. If one of
-  these signals has a listener installed, its default behaviour will be removed
+  these signals has a listener installed, its default behavior will be removed
   (io.js will no longer exit).
 - `SIGPIPE` is ignored by default, it can have a listener installed.
 - `SIGHUP` is generated on Windows when the console window is closed, and on other
   platforms under various similar conditions, see signal(7). It can have a
   listener installed, however io.js will be unconditionally terminated by
   Windows about 10 seconds later. On non-Windows platforms, the default
-  behaviour of `SIGHUP` is to terminate io.js, but once a listener has been
-  installed its default behaviour will be removed.
+  behavior of `SIGHUP` is to terminate io.js, but once a listener has been
+  installed its default behavior will be removed.
 - `SIGTERM` is not supported on Windows, it can be listened on.
 - `SIGINT` from the terminal is supported on all platforms, and can usually be
   generated with `CTRL+C` (though this may be configurable). It is not generated