Fix typo in load-examples command

[maxg203]

Noticed an incorrect command in the installation instructions that caused me the error as described in issue #6172.

Related issue: #6172
Related pull request: #6167

[codecov-io]

Codecov Report

Merging #6193 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #6193   +/-   ##
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  Coverage   76.92%   76.92%           
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  Files          47       47           
  Lines        9388     9388           
=======================================
  Hits         7222     7222           
  Misses       2166     2166

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[mistercrunch]

Yeah click made backward incompatible changes around underscore/dashes...

[mistercrunch]

Thanks for fixing the docs!

[maxg203]

@mistercrunch I wonder if issues could also crop up based on other occurences of the commands with underscores seen here... https://github.com/apache/incubator-superset/search?q=load_examples&unscoped_q=load_examples