test-lib-functions: reword "test_commit --append" docs

Reword the documentation for "test_commit --append" added in my
3373518cc8 (test-lib functions: add an --append option to test_commit,
2021-01-12).

A follow-up commit will make the "echo" part of this configurable, and
in any case saying "echo >>" rather than ">>" was redundant.

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

t/test-lib-functions.sh

@@ -172,8 +172,7 @@ debug () {
 #   --notick
 #	Do not call test_tick before making a commit
 #   --append
-#	Use "echo >>" instead of "echo >" when writing "<contents>" to
-#	"<file>"
+#	Use ">>" instead of ">" when writing "<contents>" to "<file>"
 #   --signoff
 #	Invoke "git commit" with --signoff
 #   --author <author>