Enhance birthdays konnector (#565)
Enhance birthdays konnector (#565)

* Fix typos in description and update it according to latest changes.
* Make the `tag` field optional, defaulting to selecting all contacts
when the `tag` field is empty.

Closes #492.

cozy-labs/konnectors,poupotte/konnectors,nono/konnectors,poupotte/konnectors,poupotte/konnectors,cozy-labs/konnectors,clochix/konnectors,nono/konnectors,clochix/konnectors,clochix/konnectors,nono/konnectors,cozy-labs/konnectors