Fix comments parsing for RBI::Method

[Morriar]

Before this PR, parsing a file such as this one:

# some comment
def foo(x); end

End up associating the comment to the x parameter:

def foo(
  x, # some comment
); end

This PR fixes the way we associate the comment to the method so the result is accurate:

# some comment
def foo(x); end

[egiurleo]

Woohoo much clearer now!