Fix invalid CSS vertical-align value for .fancy-datepicker-core

Summary:
vertical-align: center is not a valid value per https://developer.mozilla.org/en-US/docs/Web/CSS/vertical-align.
Likely middle was meant.

Test Plan: Read CSS docs; check output of your web browser console. (Interstingly, https://jigsaw.w3.org/css-validator/validator does not complain.)

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Differential Revision: https://we.phorge.it/D25920