Fixed parametricCurve() to use correct stop value

[just-georgeb]

parametricCurve() takes a start and stop value as inputs:

:param start: starting value of the parameter t
:param stop: final value of the parameter t

The final value in the current implementation is actually start + stop. This PR fixes it to end at the specified stop value

[codecov]

Codecov Report

Merging #477 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #477   +/-   ##
=======================================
  Coverage   93.63%   93.63%           
=======================================
  Files          30       30           
  Lines        5859     5860    +1     
  Branches      624      624           
=======================================
+ Hits         5486     5487    +1     
  Misses        234      234           
  Partials      139      139           

Impacted Files	Coverage Δ
cadquery/cq.py	88.29% <100.00%> (+0.01%)	⬆️

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[adam-urbanczyk]

Thanks @just-georgeb , good catch!