hash.c: [DOC] fix for Hash#store docs

* hash.c (rb_hash_aset): fix misleading example which may suggest
  that Hash.store will return self instead of value - Hash#store
  is returning value and update itself, as well as Hash#[]=.
  [Fix GH-715]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47576 b2dd03c8-39d4-4d8f-98ff-823fe69b080e