Link Array#union from | method

`Array#uniq` is not really related with `Array#|`, so I replaced it by
`Array#union`.

[Fix GH-1747] [Feature #14097]

From: Ana María Martínez Gómez <ammartinez@suse.de>

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64789 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

array.c

@@ -4294,7 +4294,7 @@ rb_ary_union(VALUE ary_union, VALUE ary)
  *     [ "a", "b", "c" ] | [ "c", "d", "a" ]    #=> [ "a", "b", "c", "d" ]
  *     [ "c", "d", "a" ] | [ "a", "b", "c" ]    #=> [ "c", "d", "a", "b" ]
  *
- *  See also Array#uniq.
+ *  See also Array#union.
  */
 
 static VALUE