Update poll_interval documentation

[etr2460]

The doc block says this is an int, but after looking through the code, it can actually be a float as that's what time.sleep supports. Updating so others aren't confused.

to: @bkyryliuk

[codecov]

Codecov Report

Merging #369 (fb0ba7f) into master (da047f3) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #369   +/-   ##
=======================================
  Coverage   93.36%   93.36%           
=======================================
  Files          14       14           
  Lines        1553     1553           
  Branches      167      167           
=======================================
  Hits         1450     1450           
  Misses         75       75           
  Partials       28       28           

Impacted Files	Coverage Δ
pyhive/presto.py	86.50% <ø> (ø)

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