Minor docfixes

[tcmoore3]

The docs for rowan.to_euler state that beta is between 0 and pi degrees, which is obviously a typo.

There's also a minor typo fixed in CONTRIBUTING.md.

[codecov]

Codecov Report

Merging #33 (1d92ce8) into master (02c0285) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #33   +/-   ##
=======================================
  Coverage   89.21%   89.21%           
=======================================
  Files           7        7           
  Lines         612      612           
  Branches       69       69           
=======================================
  Hits          546      546           
  Misses         42       42           
  Partials       24       24           

Impacted Files	Coverage Δ
rowan/functions.py	98.53% <ø> (ø)

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