fix root_path of load_swagger_file

[codermartin]

Given thing_doer.yml:

tags:
  - "thing_doer"
summary: "Do a thing"
description: ""
consumes:
  - "application/json"
produces:
  - "application/json"
parameters:
  - in: "body"
    name: "body"
    required: true
    schema:
      import: "models/stuff.yml"
responses:
  200:
    description: "The results of doing a thing"
    schema:
      type: "object"
      properties:
        whatever:
          type: "integer"

with a file structure as follows, the folder specs is the doc_dir:

specs/
   thing_doer.yml
   models/
      stuff.yml

before this fix:
the stuff.yml full path would be specs/thing_doer.yml/models/stuff.yml, and it's wrong.

after this fix:
the stuff.yml full path would be specs/models/stuff.yml, and it's right.

[billyrrr]

Thank you. I will merge it in a separate branch and test it first.