Bring hstore's comment into line with style of other contrib comments.

All the other ones that are primarily a new datatype say "data type for
<purpose>", so make this one similar.

contrib/hstore/hstore.control

 # hstore extension
-comment = 'store sets of (key, value) pairs'
+comment = 'data type for storing sets of (key, value) pairs'
 default_version = '1.0'
 module_pathname = '$libdir/hstore'
 relocatable = true