Fixes formatting

[vpetersson]

Summary

Minor formatting Markdown formatting.

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.

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[codecov]

Codecov Report

Merging #515 into master will increase coverage by 0.11%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #515      +/-   ##
==========================================
+ Coverage   69.11%   69.23%   +0.11%     
==========================================
  Files          15       15              
  Lines        1690     1690              
  Branches       93       93              
==========================================
+ Hits         1168     1170       +2     
+ Misses        499      498       -1     
+ Partials       23       22       -1

Impacted Files	Coverage Δ
slack/rtm/client.py	84.23% <0%> (+1.08%)	⬆️

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[RodneyU215]

Thank you @vpetersson!