[ux] 'new-domain' argument of maindomain command was confusing

data/actionsmap/yunohost.yml

@@ -1547,7 +1547,7 @@ tools:
                 - PUT /domains/main
             arguments:
                 -n:
-                    full: --new-domain
+                    full: --new-main-domain
                     help: Change the current main domain
                     extra:
                         pattern: *pattern_domain

src/yunohost/tools.py

@@ -165,7 +165,7 @@ def tools_adminpw(new_password, check_strength=True):
 
 
 @is_unit_operation()
-def tools_maindomain(operation_logger, new_domain=None):
+def tools_maindomain(operation_logger, new_main_domain=None):
     """
     Check the current main domain, or change it