Update appveyor.yml

[Jaxelr]

Added missing quotation mark.

[codecov]

Codecov Report

Merging #28 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #28   +/-   ##
=======================================
  Coverage   66.66%   66.66%           
=======================================
  Files           1        1           
  Lines           9        9           
=======================================
  Hits            6        6           
  Misses          3        3

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[thomasrockhu]

Thanks @Jaxelr !