array.c: fix typo

* array.c (rb_ary_count): [DOC] fix typo.  Array#count uses ==, not
  ===.  a question at asakusa.rb ML.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42003 b2dd03c8-39d4-4d8f-98ff-823fe69b080e