Add mention of rubyfmt in language server docs

[wadetandy]

• [n/a] The build passes
• [n/a] TSLint is mostly happy
• [n/a] Prettier has been run

[codecov]

Codecov Report

Merging #626 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #626   +/-   ##
======================================
  Coverage    5.04%   5.04%           
======================================
  Files          10      10           
  Lines         119     119           
  Branches       20      20           
======================================
  Hits            6       6           
  Misses        113     113           

Flag	Coverage Δ
#language_server_ruby	5.04% <ø> (ø)

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[wadetandy]

replaced by #628