Remove typo in ngram documentation

[miguelusque]

There is a typo in ngram() method documentation, consisting in a duplicated assignment.

[galipremsagar]

@gpucibot merge

[codecov]

Codecov Report

Merging #10859 (3a283b2) into branch-22.06 (6901b12) will increase coverage by 0.03%.
The diff coverage is n/a.

❗ Current head 3a283b2 differs from pull request most recent head 7cc10a7. Consider uploading reports for the commit 7cc10a7 to get more accurate results

@@               Coverage Diff                @@
##           branch-22.06   #10859      +/-   ##
================================================
+ Coverage         86.29%   86.32%   +0.03%     
================================================
  Files               144      144              
  Lines             22656    22656              
================================================
+ Hits              19551    19558       +7     
+ Misses             3105     3098       -7     

Impacted Files	Coverage Δ
python/cudf/cudf/core/column/string.py	88.78% <ø> (+0.12%)	⬆️
python/cudf/cudf/core/dataframe.py	93.78% <0.00%> (+0.04%)	⬆️
python/cudf/cudf/core/groupby/groupby.py	91.79% <0.00%> (+0.22%)	⬆️
python/cudf/cudf/core/column/numerical.py	96.17% <0.00%> (+0.29%)	⬆️
python/cudf/cudf/core/tools/datetimes.py	84.49% <0.00%> (+0.30%)	⬆️
python/cudf/cudf/core/column/lists.py	91.70% <0.00%> (+0.97%)	⬆️

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