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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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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