2.22: Missing label for reg_system templates

[Gregory Neil Shapiro]

templates/reg_system/main_menu_ca.html has the following change:

-                    Domain Name<B>:</B>
+                    {{MLDNSType}} Domain Name<B>:</B>

The same should be done in the following templates/reg_system/ files:

avail.html
main_menu.html
taken.html