Fix grammar: correct a/an article usage in docs

[fr0mano]

Fixes article usage based on pronunciation:

• a HMAC → an HMAC (pronounced "aitch-mac")
• a XOF → an XOF (pronounced "ex-oh-eff")
• an scrypt → a scrypt (pronounced "ess-crypt")

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.10%. Comparing base (79d647a) to head (4c58d8d).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #570   +/-   ##
=======================================
  Coverage   99.10%   99.10%           
=======================================
  Files         108      108           
  Lines       19708    19708           
=======================================
  Hits        19531    19531           
  Misses        177      177           

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