docs: fixed typo in documentation throttling.md

[ryuichi1208]

Summary

Fixed typo in documentation

https://github.com/GoogleChrome/lighthouse/blob/master/docs/throttling.md#devtools-lighthouse-panel-throttling

Related Issues/PRs

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[adamraine]

LGTM

[adamraine]

@ryuichi1208 Make sure to sign the CLA and we can merge this.

[ryuichi1208]

@googlebot I signed it!