Fix gitter channel badge

[fedefernandez]

What this does?

Fixes the Gitter channel badge. This is a temporary solution until #742 gets in place.

Checklist

• Reviewed the diff to look for typos, println and format errors.
• Updated the docs accordingly.

[codecov]

Codecov Report

Merging #743 into master will increase coverage by 0.25%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #743      +/-   ##
=========================================
+ Coverage   81.05%   81.3%   +0.25%     
=========================================
  Files          73      73              
  Lines        1161    1161              
  Branches       14      14              
=========================================
+ Hits          941     944       +3     
+ Misses        220     217       -3

Impacted Files	Coverage Δ
...kindness/mu/rpc/idlgen/avro/AvroSrcGenerator.scala	75% <0%> (+5%)	⬆️

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