Fix error in regexp generation for unknown spec type errors

When confronted with this type spec problem:

    diff --git a/src/riak_kv_vnode_status_mgr.erl b/src/riak_kv_vnode_status_mgr.erl
    index 33235b9..5f56121 100644
    --- a/src/riak_kv_vnode_status_mgr.erl
    +++ b/src/riak_kv_vnode_status_mgr.erl
    @@ -271,7 +271,7 @@ read_vnode_status(File) ->
     %% @private write the vnode status. This is why the file is guarded by
     %% the process. This file should have no concurrent access, and MUST
     %% not be written at any other place/time in the system.
    --spec write_vnode_status(status(), file:filname()) -> ok.
    +-spec write_vnode_status(status(), file:filename()) -> ok.
     write_vnode_status(Status, File) ->
         VersionedStatus = replace(version, ?VNODE_STATUS_VERSION, Status),
         riak_core_util:replace_file(File, io_lib:format("~p.", [VersionedStatus])).

... the unpatched version of the sed hackery in `tools.mk` creates
the following:

    % cat dialyzer_unhandled_warnings
    ^file\:filname/0$

... which does not match the detected error.  Note the two spaces before
the "f" in `file`:

    % grep :filname dialyzer_warnings
      file:filname/0

This patch adds a spec for optional whitespace at the beginning of the line.