vignettes: avoid using _ or . in header IDs

[aitap]

"Introduction to data.table" on CRAN seems to be the only vignette affected:

<h3 id="h-great-but-how-can-i-refer-to-columns-by-names-in-j-like-in-a-data-frame" #refer_j>h) Great! But how can I refer to columns by names in <code>j</code> (like in a <code>data.frame</code>)?</h3>

<h4 id="how-can-we-calculate-the-number-of-trips-for-each-origin-airport-for-carrier-code-quot-aa-quot" #origin-.N>– How can we calculate the number of trips for each origin airport for carrier code <code>&quot;AA&quot;</code>?</h4>

<h4 id="how-can-we-get-the-total-number-of-trips-for-each-origin-dest-pair-for-carrier-code-quot-aa-quot" #origin-dest-.N>– How can we get the total number of trips for each <code>origin, dest</code> pair for carrier code <code>&quot;AA&quot;</code>?</h4>

As a result, the links to these headers don't work.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.60%. Comparing base (32dfd8d) to head (2916c6c).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6784   +/-   ##
=======================================
  Coverage   98.60%   98.60%           
=======================================
  Files          79       79           
  Lines       14642    14642           
=======================================
  Hits        14438    14438           
  Misses        204      204           

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[MichaelChirico]

Want to have a go at writing a simple grep() linter for this under code-quality/lint-md?

https://github.com/Rdatatable/data.table/tree/master/.ci/linters/md

[MichaelChirico]

Feel free to merge before/after adding a linter; if you'd rather punt on the linter, please file a follow-up.

[aitap]

Pardonnez-moi @Rdatatable/french, I hope you don't mind me fixing the heading ids and the links without changing the text.