codecov: update to project threshold to explicit %

[Jusshersmith]

Problem

Codecov appears to be incorrectly failing some pull request checks due to the project status check. If I understand correctly, our Codecov configuration should mean that the project check should fail if the change results in a 2% drop in overall coverage.

#279 is failing due to a 0.47% drop in project coverage.

Solution

It seems the Codecov docs have been updated to show the threshold being set explicitly as a %, and some past issues seem to suggest that this may affect how it works, e.g. https://community.codecov.io/t/threshold-confusion/88.
It's unclear if this is the cause, but seems sensible to update regardless and to rule it out.

[codecov]

Codecov Report

Merging #283 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #283   +/-   ##
=======================================
  Coverage   61.85%   61.85%           
=======================================
  Files          57       57           
  Lines        4638     4638           
=======================================
  Hits         2869     2869           
  Misses       1556     1556           
  Partials      213      213