Correct spelling in index.md

[galenseilis]

Changed spelling from "inalid" to "invalid".

[deepyaman]

I noticed this the other day and forgot to raise a PR. Much appreciated!

I went ahead and set DCO to pass for you; in the future, you can sign off commits using the --signoff flag on git commit.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.23%. Comparing base (812b2a8) to head (b0341b3).
Report is 148 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1819      +/-   ##
==========================================
- Coverage   94.28%   93.23%   -1.05%     
==========================================
  Files          91      120      +29     
  Lines        7013     9155    +2142     
==========================================
+ Hits         6612     8536    +1924     
- Misses        401      619     +218     

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