Stack Exchange sites do all have the following page in the help center:
Help Center → Asking → What topics can I ask about here?
The last paragraph contains the following sentence (emphasis mine):
[...] If no site currently exists that will accept your question, you may commit to or propose a new site at Area51, the place where new Stack Exchange communities are democratically created.
In the Guidelines for the Use of the Stack Exchange Trademarks which can be found here, it is mentioned that Area 51 is written in two words (emphasis as in source):
Area 51 (two words) is the site used to propose new Stack Exchange sites for the Stack Exchange Network.
Given the referenced guidelines, I think the help pages in question across the whole Stack Exchange Network should be updated to use the correct form of Area 51 (two words).