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.
Update copy in "Control Appearance on Social Sites"

Fix several small typos in this page, including the one reported by
@coonsta.

Fixes #992.
Merge pull request #993 from jacquerie/issue-992

Update copy in "Control Appearance on Social Sites"
Merge pull request #990 from jacquerie/issue-989

Fix typo in category title
Consolidate clean and canonical_url

Create a new filter called `canonicalize`, and replace every
instance of `clean` and `canonical_url` with it.

Fixes #984.
Merge branch 'master' of github.com:google/WebFundamentals into issue-984