docs(readme): refine prettier examples

[JLHwung]

Revised the following points:

• Added stylelint --fix on "Reformatting the code" section
• Simplified prettier calls since it can infer parser type from the file extension
• Added prettier example on Markdown and HTML files

[codecov]

Codecov Report

Merging #541 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #541   +/-   ##
=====================================
  Coverage      98%    98%           
=====================================
  Files          13     13           
  Lines         351    351           
  Branches       44     44           
=====================================
  Hits          344    344           
  Misses          7      7

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[okonet]

Thanks!