Docs: change Dict.merge description to read more naturally

Previously there was an additional "dict" - this is as a result of it
previously saying "dict `a`"

Apologies for the small pull request - this issue came up when being reviewed. @khia - could you just leave a note on this pull request to confirm there are no other documentation issues in the https://github.com/elixir-lang/elixir/pull/2575 pull request?