Use correct spelling of statistics kind

A couple error messages and comments used 'statistic kind', not the
correct 'statistics kind'. Fix and backpatch all the way back to 10,
where extended statistics were introduced.

Backpatch-through: 10

src/include/nodes/pathnodes.h

@@ -923,7 +923,7 @@ typedef struct StatisticExtInfo
 
 	Oid			statOid;		/* OID of the statistics row */
 	RelOptInfo *rel;			/* back-link to statistic's table */
-	char		kind;			/* statistic kind of this entry */
+	char		kind;			/* statistics kind of this entry */
 	Bitmapset  *keys;			/* attnums of the columns covered */
 } StatisticExtInfo;