[Tagging] spelling of healthcare=birthing_center

[Timeo Gut]

Hi all,

Currently the documentation for healthcare=birthing_center uses American 
spelling. This is very prone to errors given general conventions and the 
fact that there are many other tags that use the British spelling of the 
same word (vaccination_centre, sports_centre, community_centre).

There are only 427 uses of this tag. More than 5% of them already have 
British spelling.

I'm considering updating the wiki and then re-tagging the remaining uses 
of the American variant.

Best regards,
Timmy