Fix IndexInfo comments.

Recently, ii_KeyAttrNumbers was renamed to ii_IndexAttrNumbers, and ii_Am
field was added, but the comments were not updated.

Author: Yugo Nagata
Discussion: https://www.postgresql.org/message-id/20180830134831.e35a91b8b978b248c16c8f7b@sraoss.co.jp

src/include/nodes/execnodes.h

@@ -120,7 +120,7 @@ typedef struct ExprState
  *
  *		NumIndexAttrs		total number of columns in this index
  *		NumIndexKeyAttrs	number of key columns in index
- *		KeyAttrNumbers		underlying-rel attribute numbers used as keys
+ *		IndexAttrNumbers	underlying-rel attribute numbers used as keys
  *							(zeroes indicate expressions). It also contains
  * 							info about included columns.
  *		Expressions			expr trees for expression entries, or NIL if none
@@ -138,6 +138,7 @@ typedef struct ExprState
  *		Concurrent			are we doing a concurrent index build?
  *		BrokenHotChain		did we detect any broken HOT chains?
  *		ParallelWorkers		# of workers requested (excludes leader)
+ *		Am					Oid of index AM
  *		AmCache				private cache area for index AM
  *		Context				memory context holding this IndexInfo
  *