Move Freezing Cloud to level 5

Since clouds now dissipate out of LOS and Ozocubu's Refrigeration
now deals heavy damae, it's hard to justify the investment in Freezing
Cloud as a 3-school spell. Move it to level 5 to make it a bit more
viable, and to add symmetry with Bolt of Magma as a L5 Fire/Earth/Cj.

crawl-ref/source/spl-data.h

@@ -268,7 +268,7 @@ static const struct spell_desc spelldata[] =
     spschool::conjuration | spschool::ice | spschool::air,
     spflag::target | spflag::area | spflag::needs_tracer
         | spflag::cloud,
-    6,
+    5,
     200,
     5, 5,
     6, 2,