docs: fix npm install cmd in readme

[JustinBeckwith]

Fixes #<issue_number_goes_here> (it's a good idea to open an issue first for discussion)

• Tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

[codecov]

Codecov Report

Merging #1635 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1635   +/-   ##
=======================================
  Coverage   98.46%   98.46%           
=======================================
  Files           2        2           
  Lines         260      260           
  Branches        2        2           
=======================================
  Hits          256      256           
  Misses          4        4

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