Fix a typo in decode.c

Hi,

When reading the code, I noticed a typo in the description of WAL record.

/*
- * Decode XLOG_HEAP2_MULTI_INSERT_insert record into multiple tuplebufs.
+ * Decode XLOG_HEAP2_MULTI_INSERT record into multiple tuplebufs.
  *

And attach a small patch to fix it.

Best Regards,
Hou Zhijie