Fix typo in category title

As reported by @Vanvox, title should say 'Formen' instead of
'Formrn'.

Fixes #989.
Update typo correction

@Vanvox reports that the correct word wasn't 'Formen' anyway,
but 'Formulare'.

Fixes #989.
Merge pull request #990 from jacquerie/issue-989

Fix typo in category title