config: Simplify title to "Configuration"

The capitalization in "Container Configuration file" (which we've used
since 70372d3, *.md: update TOC and links, 2015-09-10, opencontainers#176) was
halfway between "Title Case" and "Sentence case".  The current spec
isn't particularly consistent (e.g. we have both "Specification
version" and "POSIX-platform Mounts"), but the ToC has used
"Configuration" for this file since e7be40f (Cleanup the spec a bit
to remove WG/git text that's not really part of the spec, 2016-11-14,
opencontainers#626) so dodge the sentence/title issue and use that here too.

Signed-off-by: W. Trevor King <wking@tremily.us>

config.md

@@ -1,4 +1,4 @@
-# <a name="containerConfigurationFile" />Container Configuration file
+# <a name="configuration" />Configuration
 
 This configuration file contains metadata necessary to implement [standard operations](runtime.md#operations) against the container.
 This includes the process to run, environment variables to inject, sandboxing features to use, etc.