Fix incorrectly reported stats kind in "can't happen" ERROR

The error message(s) were reporting the stats kind of 'f', which is not
correct as that's for the "dependencies" statistics kind.

Reported-by: Horst Reiterer
Reviewed-by: Richard Guo
Discussion: https://postgr.es/m/18375-ba99383eb9062d6a@postgresql.org
Backpatch-through: 12, where MCV extended stats were added.

src/backend/statistics/extended_stats.c

@@ -2480,7 +2480,7 @@ statext_expressions_load(Oid stxoid, int idx)
 	if (isnull)
 		elog(ERROR,
 			 "requested statistics kind \"%c\" is not yet built for statistics object %u",
-			 STATS_EXT_DEPENDENCIES, stxoid);
+			 STATS_EXT_EXPRESSIONS, stxoid);
 
 	eah = DatumGetExpandedArray(value);
 

src/backend/statistics/mcv.c

@@ -575,7 +575,7 @@ statext_mcv_load(Oid mvoid)
 	if (isnull)
 		elog(ERROR,
 			 "requested statistics kind \"%c\" is not yet built for statistics object %u",
-			 STATS_EXT_DEPENDENCIES, mvoid);
+			 STATS_EXT_MCV, mvoid);
 
 	result = statext_mcv_deserialize(DatumGetByteaP(mcvlist));