docs: Tweak appveyor badge

[sudo-suhas]

Right now, we have 2 build badges which look exactly the same:

And it bothers me just a little bit 😅. This PR tweaks the appveyor badge as shown below.

Appveyor docs - https://www.appveyor.com/docs/status-badges/

[codecov]

Codecov Report

Merging #316 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #316   +/-   ##
=======================================
  Coverage   95.95%   95.95%           
=======================================
  Files          10       10           
  Lines         173      173           
  Branches       27       27           
=======================================
  Hits          166      166           
  Misses          7        7

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