docs: fix typo in env var name

[alekseik1]

Hi! A small typo in docs here.

I made it PANDERA_VALIDATION_ENABLED everywhere on the page.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.11%. Comparing base (4df61da) to head (f63e3e6).
Report is 57 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #1562       +/-   ##
===========================================
- Coverage   94.29%   83.11%   -11.18%     
===========================================
  Files          91      111       +20     
  Lines        7024     8203     +1179     
===========================================
+ Hits         6623     6818      +195     
- Misses        401     1385      +984     

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