Remove ngrams argument examples from dfm help

[kbenoit]

Removes the examples from the dfm help that still contain the ngrams argument, despite this being removed many versions ago.

Solves #1990
Closes #1896

See also #1602

[codecov]

Codecov Report

Merging #2005 (38100b3) into master (9182963) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2005   +/-   ##
=======================================
  Coverage   90.15%   90.15%           
=======================================
  Files         102      102           
  Lines        6512     6512           
=======================================
  Hits         5871     5871           
  Misses        641      641           

Impacted Files	Coverage Δ
R/dfm.R	98.65% <ø> (ø)

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