docs/config: consistify include.path examples

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 <redacted>
Reviewed-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>