Fix typo in SP-GiST error message

Error message didn't match the actual check.  Fix that.  Compression of leaf
SP-GiST values was introduced in 11.  So, backpatch.

Discussion: https://postgr.es/m/20180810.100742.15469435.horiguchi.kyotaro%40lab.ntt.co.jp
Author: Kyotaro Horiguchi
Backpatch-through: 11

src/backend/access/spgist/spgutils.c

@@ -133,7 +133,7 @@ spgGetCache(Relation index)
 			if (!OidIsValid(index_getprocid(index, 1, SPGIST_COMPRESS_PROC)))
 				ereport(ERROR,
 						(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
-						 errmsg("compress method must not defined when leaf type is different from input type")));
+						 errmsg("compress method must be defined when leaf type is different from input type")));
 
 			fillTypeDesc(&cache->attLeafType, cache->config.leafType);
 		}