Renaming variable is not possible from `-spec` attribute

[robertoaloi]

Describe the bug

While it's possible to rename a variable via the a textDocument/rename request in the context of a function, this is not possible from a -spec attribute.

To Reproduce**

Have code such as:

-spec handle_cast(Request, state()) -> Result when
    Request :: cast_request(),
    Result :: {noreply, state()} | {stop, shutdown, state()}.
handle_cast(_, _State) ->
    error(not_implemented).

Expected behavior

Put the cursor over Request, press F2 (or the equivalent shortcut for renaming), rename to Message and have all occurrences in the spec changed.

Actual behavior

The rename operation fails with an error