docs/config: consistify include.path examples

Most of the include examples use "foo.inc", but some use
"foo". Since the string of examples are meant to show
variations and how they differ, it's a good idea to change
only one thing at a time. The filename differences are not
relevant to what we're trying to show.

Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: peff

Documentation/config.txt

@@ -172,8 +172,8 @@ Example
 
 	[include]
 		path = /path/to/foo.inc ; include by absolute path
-		path = foo ; find "foo" relative to the current file
-		path = ~/foo ; find "foo" in your `$HOME` directory
+		path = foo.inc ; find "foo.inc" relative to the current file
+		path = ~/foo.inc ; find "foo.inc" in your `$HOME` directory
 
 	; include if $GIT_DIR is /path/to/foo/.git
 	[includeIf "gitdir:/path/to/foo/.git"]