Rename "Users guide" everywhere by "User Guide"

[jonquet]

I don't really know how we end up with "users guide" which is not really gramitacally correct.
Plus the capital letter is missing and its niot consistent with the other Guide name.

I suggest to change the title to "User Guide". I don't do it as it touches 278 references in the whole project.

[graybeal]

No more grammatically incorrect than User Guide. But I agree that uniformity and camel case are both worthy improvements, and people are more used to User Guide.