DOC: use https in css stylesheet url

[kaczmarj]

Summary

This PR proposes using https instead of http for the stylesheet https://www.google.com/cse/style/look/default.css. Currently, Chrome 70.0.3538.102 on macOS blocks that stylesheet from being loaded via http, with the error:

Mixed Content: The page at 'https://nipype.readthedocs.io/en/latest/index.html' was loaded over HTTPS, but requested an insecure stylesheet 'http://www.google.com/cse/style/look/default.css'. This request has been blocked; the content must be served over HTTPS.

List of changes proposed in this PR (pull-request)

• Use https instead of http for stylesheet

Acknowledgment

• (Mandatory) I acknowledge that this contribution will be available under the Apache 2 license.