Fix typos in README

[Joe-Downs]

The README had a few typos.

1. Using the contraction form of it's rather than the possessive its.
2. A period missing before a capital. (I assume they should be two separate sentences.)
3. which are basically buttons on the checks report is a non-essential clause, so I added a comma to the beginning.

---

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[codecov]

Codecov Report

Merging #219 (9176f2a) into master (7aeccf4) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #219   +/-   ##
=========================================
  Coverage     85.38%   85.38%           
  Complexity      152      152           
=========================================
  Files            17       17           
  Lines           746      746           
  Branches         57       57           
=========================================
  Hits            637      637           
  Misses           89       89           
  Partials         20       20           

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[uhafner]

Thanks!