Set codecov failure threshold to 0.2%

[StanczakDominik]

As per SunPy's codecov configuration.

• I have added a changelog entry for this pull request (please see
  changelog/README.rst for instructions, and if you need help with picking the PR type, ask!)
• If adding new functionality, I have added (passing) tests and
  docstrings. (Tests pop up at the bottom, in the checks box).
• I have fixed any new failing tests (if you're unsure why
  they're failing, ask!).

[codecov]

Codecov Report

Merging #704 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #704   +/-   ##
=======================================
  Coverage   95.04%   95.04%           
=======================================
  Files          57       57           
  Lines        4704     4704           
=======================================
  Hits         4471     4471           
  Misses        233      233

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