[docs] Fix Markdown syntax typo in Webpack docs.

[jessestuart]

Summary
A un-escaped reference to <rootDir> in a blockquote was being interpreted as an HTML tag, causing it not to appear in the rendered docs — the result of which is a slightly confusing read. This PR surrounds the tag in backticks to ensure it's rendered correctly. (See screenshots below)

Test plan

I've verified this fix by running the docs website locally, e.g.:

$ yarn && yarn build
$ cd website && yarn start # Verify regression exists in `master` (see screenshot #1)
$ cd -
$ nvim docs/Webpack.md  # Fix typo
$ cd website && yarn start # Verify regression is fixed (see screenshot #2)

Screenshot #1

Screenshot #2

[codecov-io]

Codecov Report

Merging #5280 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #5280   +/-   ##
=======================================
  Coverage   60.98%   60.98%           
=======================================
  Files         203      203           
  Lines        6816     6816           
  Branches        3        4    +1     
=======================================
  Hits         4157     4157           
  Misses       2658     2658           
  Partials        1        1

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[github-actions]

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