Fix some comments in xlogreader.h
segment_open and segment_close were mentioned with incorrect names.
Discussion:
* XLogReadRecord or XLogFindNextRecord; it can be passed in as NULL
* otherwise. The WALRead function can be used as a helper to write
* page_read callbacks, but it is not mandatory; callers that use it,
- * must supply open_segment callbacks. The close_segment callback
+ * must supply segment_open callbacks. The segment_close callback
* must always be supplied.
*
* After reading a record with XLogReadRecord(), it's decomposed into
