Fix comment in CheckIndexCompatible().

Commit 5f173040 removed the parameter "heapRelation" from
CheckIndexCompatible(), but forgot to remove the mention of it
from the comment. This commit removes that unnecessary mention.

Also this commit adds the missing mention of the parameter "oldId"
in the comment.

Author: Yugo Nagata
Reviewed-by: Nathan Bossart, Fujii Masao
Discussion: https://postgr.es/m/20220204014634.b39314f278ff4ae3de96e201@sraoss.co.jp

src/backend/commands/indexcmds.c

@@ -129,7 +129,7 @@ typedef struct ReindexErrorInfo
  *		prospective index definition, such that the existing index storage
  *		could become the storage of the new index, avoiding a rebuild.
  *
- * 'heapRelation': the relation the index would apply to.
+ * 'oldId': the OID of the existing index
  * 'accessMethodName': name of the AM to use.
  * 'attributeList': a list of IndexElem specifying columns and expressions
  *		to index on.