t3427: adjust stale comment

In b6211b89eb (tests: avoid variations of the `master` branch name,
2020-09-26), the `master[123]` branch names were renamed to
`topic_[123]`. A non-literal mention of the corresponding files was
missed in that commit.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3427-rebase-subtree.sh

@@ -19,7 +19,7 @@ commit_message() {
 #                             \
 # README ---------------------- Add subproject master - topic_4 - files_subtree/topic_5
 #
-# Where the merge moves the files master[123].t into the subdirectory
+# Where the merge moves the files topic_[123].t into the subdirectory
 # files_subtree/ and topic_4 as well as files_subtree/topic_5 add files to that
 # directory directly.
 #