fix broken markdown

[kimpham54]

GitHub Issue: fix broken markdown

• Other Relevant Links (Google Groups discussion, related pull requests, Release pull requests, etc.)

What does this Pull Request do?

Fix headings in README

What's new?

• fix syntax to render headings

How should this be tested?

• Review README

Additional Notes:

Interested parties

@ruebot

[codecov]

Codecov Report

Merging #65 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #65   +/-   ##
=======================================
  Coverage   93.45%   93.45%           
=======================================
  Files           3        3           
  Lines         107      107           
=======================================
  Hits          100      100           
  Misses          7        7

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[ruebot]

Thanks @kimpham54!