chore: fix activationkey messaging in man page

The previous instruction message for registering
a system with an activation key suggested that
a username and password was not neccesary, but possible,
which is not the case.

Card ID: CCT-545

man/subscription-manager.8

@@ -227,7 +227,7 @@ Gives a comma-separated list of product keys to apply specific subscriptions to
 .IP
 When the
 .B --activationkey
-option is used, it is not necessary to use the
+option is used, it is not possible to use the
 .B --username
 and
 .B --password