Free column naming inconsistent in specs vs. implementation

Environment

• OS version: n/a
• Browser version: n/a
• Extension version: release branch
• Last working version: n/a

Steps to reproduce

1. Install ABP
2. Open options page

Observed behavior

In specs, the headline for the column with free features should be FREE FEATURES, see https://gitlab.com/adblockinc/ext/adblockplus/spec/-/blob/c72d0084ba63427aebf4d8da461405aad726fc6f/spec/abp/desktop-settings/general-tab.md#free-features-section-headline

In designs and implementation, the headline is FREE EXTRAS

Expected behavior

Either update the specs or the headline in the extension.

Further information

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Hints for Translators

• Change string value of "options_free_filters_header"