Fix bug in freemonad doc

[ceedubs]

Thanks to @tek for pointing out that the "pure" State-based compiler
was using the mutable map from the "impure" example.

[codecov-io]

Current coverage is 89.34%

Merging #878 into master will not affect coverage as of 1286632

@@            master    #878   diff @@
======================================
  Files          169     169       
  Stmts         2337    2337       
  Branches        75      75       
  Methods          0       0       
======================================
  Hit           2088    2088       
  Partial          0       0       
  Missed         249     249       

Review entire Coverage Diff as of 1286632

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[adelbertc]

👍 , merging because doc