Fix function spec find bug.

It is okay to write smth like:
```erlang
-spec foo() -> atom().
%% other stuff
spec bar() -> atom().
bar() -> ok.
%% other stuff
foo() -> ok.
```
in terms of Erlang interpreter, i.e. specify spec not right before function definition.
But in this case ErlangPsiImplUtil#getSpecification doesn't see spec for foo (as you can see while "Remove function" intention).
This is fixed now.
That's wrong. Please see org.intellij.erlang.psi.ErlangSpecFun#getReference and isReferenceTo() method.