fix: PULL_REQUEST_TEMPLATE.md old url

[weiyichao]

What's the purpose of this PR

Update the link address of the contribution guide and change document in the PR template.

Old link

  https://github.com/ctripcorp/apollo/blob/master/CONTRIBUTING.md
  https://github.com/ctripcorp/apollo/blob/master/CHANGES.md

Replace with new link

  https://github.com/apolloconfig/apollo/blob/master/CONTRIBUTING.md
  https://github.com/apolloconfig/apollo/blob/master/CHANGES.md

Follow this checklist to help us incorporate your contribution quickly and easily:

• Read the Contributing Guide before making this pull request.
• Write a pull request description that is detailed enough to understand what the pull request does, how, and why.
• Update the CHANGES log.

[codecov]

Codecov Report

Merging #4726 (d7b034d) into master (60bd514) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master    #4726   +/-   ##
=========================================
  Coverage     47.21%   47.21%           
  Complexity     1660     1660           
=========================================
  Files           349      349           
  Lines         10691    10691           
  Branches       1062     1062           
=========================================
  Hits           5048     5048           
  Misses         5335     5335           
  Partials        308      308           

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[nobodyiam]

Thanks!