Correcting the spelling of the principal username in the Presto client.

[Ralnoc]

Correcting the spelling in the presto.py from principle_username to principal_username.

Please let me know if we need to retain the original variable for backwards compatibility and I'll update the PR accordingly.

[codecov]

Codecov Report

Merging #326 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #326   +/-   ##
=======================================
  Coverage   93.25%   93.25%           
=======================================
  Files          14       14           
  Lines        1526     1526           
  Branches      165      165           
=======================================
  Hits         1423     1423           
  Misses         75       75           
  Partials       28       28           

Impacted Files	Coverage Δ
pyhive/presto.py	85.98% <100.00%> (ø)

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[bkyryliuk]

@Ralnoc this is good enough. I'll add note to the release notes.