fix R cmd check note due to importance_sampling roxygen template

[jgabry]

importance_sampling has argument method but was using a roxygen template for argument is_method leading to an r cmd check note.

[codecov-commenter]

Codecov Report

Merging #233 (9df7236) into master (69fa2f6) will not change coverage.
The diff coverage is 28.57%.

❗ Current head 9df7236 differs from pull request most recent head b85d1eb. Consider uploading reports for the commit b85d1eb to get more accurate results

@@           Coverage Diff           @@
##           master     #233   +/-   ##
=======================================
  Coverage   92.96%   92.96%           
=======================================
  Files          30       30           
  Lines        2770     2770           
=======================================
  Hits         2575     2575           
  Misses        195      195           

Files	Coverage Δ
R/importance_sampling.R	84.04% <28.57%> (ø)

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