Improve slightly misleading internal error message

The error message was talking about RowCompareType but was actually
checking strategy numbers.  While those are closely related, it is
better to be accurate.

Discussion: https://www.postgresql.org/message-id/flat/E72EAA49-354D-4C2E-8EB9-255197F55330@enterprisedb.com

src/backend/access/nbtree/nbtutils.c

@@ -4033,8 +4033,7 @@ _bt_check_rowcompare(ScanKey skey, IndexTuple tuple, int tupnatts,
 			result = (cmpresult > 0);
 			break;
 		default:
-			elog(ERROR, "unrecognized RowCompareType: %d",
-				 (int) subkey->sk_strategy);
+			elog(ERROR, "unexpected strategy number %d", subkey->sk_strategy);
 			result = 0;			/* keep compiler quiet */
 			break;
 	}