update readme file

[isacikgoz]

Summary

We should not use -u flag when installing mmctl. From the documentation:

The -u flag instructs get to update modules providing dependencies
of packages named on the command line to use newer minor or patch
releases when available.

This can cause some pain when trying to install if there are new versions of the dependencies.

Example: https://community-daily.mattermost.com/core/pl/dcfrp6nzptyuz89q5uziwcu9ic

[codecov-commenter]

Codecov Report

Merging #404 (bc7f696) into master (2097bbf) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #404   +/-   ##
=======================================
  Coverage   67.17%   67.17%           
=======================================
  Files          45       45           
  Lines        4619     4619           
=======================================
  Hits         3103     3103           
  Misses       1315     1315           
  Partials      201      201           

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