[ngrams] small simplification to docNgrams function

4 jobs for 224-dev-understanding-ngrams in 47 minutes and 11 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Cabal
passed #16640
cabal

00:19:06

 
  Stack
passed #16641
stack

00:07:03

 
  Bench
manual #16642
allowed to fail manual
bench
 
  Test
passed #16643
test

00:21:00