Fix two doxygen warnings caused by the presence of .md in the URL

[mcol]

Summary

Following on from #1544, this fixes these two more warnings:

contributor_help_pages/developer_doc.md:42: warning: unable to resolve reference to `https:' for \ref command
README.md:8: warning: unable to resolve reference to `https:' for \ref command

These were caused by linking to urls that ended with .md, which is one of the extensions that doxygen tests for documents to parse.

By using the URL encoding symbol %2E instead of ., we can avoid the warning while keeping the URL still functional: https://github.com/stan-dev/math/blob/develop/LICENSE%2Emd

Tests

None.

Side Effects

None

Checklist

• Math issue a couple doxygen errors on develop #1543

• Copyright holder: Marco Colombo

  The copyright holder is typically you or your assignee, such as a university or company. By submitting this pull request, the copyright holder is agreeing to the license the submitted work under the following licenses:
  - Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
  - Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)

• the basic tests are passing

  • docs build, (make doxygen)

• the new changes are tested

[rok-cesnovar]

Thanks!