doc: clarify macosx-firewall suggestion BUILDING

`./tools/macosx-firewall.sh` fails if run before build step. Since the
suggestion comes before the build steps in the document, this change
clarifies that the script should be run after building.

PR-URL: #10311
Reviewed-By: Myles Borins <myles.borins@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Italo A. Casas <me@italoacasas.com>
Reviewed-By: Gibson Fahnestock <gibfahn@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

BUILDING.md

@@ -24,8 +24,8 @@ On OS X, you will also need:
     this under the menu `Xcode -> Preferences -> Downloads`
   * This step will install `gcc` and the related toolchain containing `make`
 
-* You may want to setup [firewall rules](tools/macosx-firewall.sh) to avoid
-popups asking to accept incoming network connections when running tests:
+* After building, you may want to setup [firewall rules](tools/macosx-firewall.sh)
+to avoid popups asking to accept incoming network connections when running tests:
 
 ```console
 $ sudo ./tools/macosx-firewall.sh