Some lines in the "tree" class documentation edited

[ritvik06]

1. leaves_iterator() replaced with python function leaves()

2. order for positional arguments for hg.accumulate_parallel() and hg.accumulate_parallel() in sample code was given wrong

[codecov[bot]]

Codecov Report

Merging #196 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #196   +/-   ##
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  Coverage   98.80%   98.80%           
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  Files          43       43           
  Lines        4345     4345           
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  Hits         4293     4293           
  Misses         52       52           

[PerretB]

Thanks a lot!