Comment fix: "buffer context lock" to "buffer content lock".

The term "buffer context lock" is outdated as of commit 5d5087363d.

src/backend/access/transam/xact.c

@@ -2830,7 +2830,7 @@ AbortTransaction(void)
 	pgstat_report_wait_end();
 	pgstat_progress_end_command();
 
-	/* Clean up buffer context locks, too */
+	/* Clean up buffer content locks, too */
 	UnlockBuffers();
 
 	/* Reset WAL record construction state */