Add missing opening bracket

[fbunsmann]

Fix small syntactical error in code example for custom operator in FE filter bar

[cla-assistant]

All committers have signed the CLA.

[KvM2]

Hi @fbunsmann, thanks for noticing this detail! We'll take a look at it and get back to you.

[FrantisekZak]

Added the missing bracket. The change will appear online with the 1.444 release.

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