[GEARPUMP-344] Fix broken links in README

[manuzhang]

Be sure to do all of the following to help us incorporate your contribution
quickly and easily:

• Make sure the commit message is formatted like:
  [GEARPUMP-<Jira issue #>] Meaningful description of pull request
• Make sure tests pass via sbt clean test.
• Make sure old documentation affected by the pull request has been updated and new documentation added for new functionality.

[manuzhang]

R: @huafengw

[codecov-io]

Codecov Report

Merging #240 into master will decrease coverage by 0.04%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #240      +/-   ##
==========================================
- Coverage    69.1%   69.05%   -0.05%     
==========================================
  Files         191      191              
  Lines        6124     6124              
  Branches      539      539              
==========================================
- Hits         4232     4229       -3     
- Misses       1892     1895       +3

[huafengw]

+1