Use relative path for api.specs_url.

[jslay88]

This resolves any issues with mixed content when running behind an HTTPS reverse proxy.
There is no valid use case for using absolute over relative here, and only causes additional issues.

Resolves #188

[codecov]

Codecov Report

Merging #214 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #214   +/-   ##
=======================================
  Coverage   96.92%   96.92%           
=======================================
  Files          19       19           
  Lines        2697     2697           
=======================================
  Hits         2614     2614           
  Misses         83       83           

Impacted Files	Coverage Δ
flask_restx/api.py	97.13% <100.00%> (ø)

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[ziirish]

Thanks for your contribution!