Fixed broken link to pandas dtypes

[ericmjl]

As per title, nothing biggie.

[codecov-commenter]

Codecov Report

Merging #264 into master will increase coverage by 0.16%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #264      +/-   ##
==========================================
+ Coverage   96.62%   96.78%   +0.16%     
==========================================
  Files          15       15              
  Lines        1332     1492     +160     
==========================================
+ Hits         1287     1444     +157     
- Misses         45       48       +3     

Impacted Files	Coverage Δ
pandera/decorators.py	90.80% <0.00%> (-1.74%)	⬇️
pandera/errors.py	100.00% <0.00%> (ø)
pandera/checks.py	98.60% <0.00%> (+0.21%)	⬆️
pandera/schema_components.py	98.97% <0.00%> (+0.56%)	⬆️
pandera/hypotheses.py	94.11% <0.00%> (+1.66%)	⬆️

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