test-lib-functions.sh: fix usage for test_commit()

The usage comment for test_commit() shows that the --author option
should be given as `--author=<author>`. However, this is incorrect as it
only works when given as `--author <author>`. Correct this erroneous
text.

Also, for the sake of correctness, fix the description as well since we
invoke `git commit` with `--author <author>`, not `--author=<author>`.

Signed-off-by: Denton Liu <[email protected]>
Acked-by: Ævar Arnfjörð Bjarmason <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: Denton-L

t/test-lib-functions.sh

@@ -188,8 +188,8 @@ debug () {
 #	"<file>"
 #   --signoff
 #	Invoke "git commit" with --signoff
-#   --author=<author>
-#	Invoke "git commit" with --author=<author>
+#   --author <author>
+#	Invoke "git commit" with --author <author>
 #
 # This will commit a file with the given contents and the given commit
 # message, and tag the resulting commit with the given tag name.