fix(typemap): .md -> .html

[avivkeller]

Parity with https://github.com/nodejs/node/blob/main/doc/type-map.json
Fixes: nodejs/undici-website#24

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.48%. Comparing base (8453a77) to head (6d8cd9c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5602      +/-   ##
==========================================
- Coverage   93.49%   93.48%   -0.01%     
==========================================
  Files         110      110              
  Lines       38281    38281              
==========================================
- Hits        35789    35788       -1     
- Misses       2492     2493       +1     

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