Fix obsolete comment.

Commit 3bd261ca should have updated
this, but didn't.

Extracted from a larger patch by Piotr Stefaniak.

src/backend/storage/ipc/shm_mq.c

@@ -103,7 +103,7 @@ struct shm_mq
  * locally by copying the chunks into a backend-local buffer.  mqh_buffer is
  * the buffer, and mqh_buflen is the number of bytes allocated for it.
  *
- * mqh_partial_message_bytes, mqh_expected_bytes, and mqh_length_word_complete
+ * mqh_partial_bytes, mqh_expected_bytes, and mqh_length_word_complete
  * are used to track the state of non-blocking operations.  When the caller
  * attempts a non-blocking operation that returns SHM_MQ_WOULD_BLOCK, they
  * are expected to retry the call at a later time with the same argument;