Fix typo: "can not" => "cannot"

- Noticed when doing enUS to enGB on Weblate of the Weblate software (how meta...).
- Technically, "can not" is valid, but "cannot" is used more frequently and "can not" with the space is generally used when not able to be a contraction like "can't". In this case, use the version without the space because "You can't change demo account on the demo server" is valid.

weblate/accounts/views.py

@@ -128,7 +128,7 @@ def deny_demo(request):
     """
     messages.warning(
         request,
-        _('You can not change demo account on the demo server.')
+        _('You cannot change demo account on the demo server.')
     )
     return redirect('profile')