Fix misleading :add docs in user guide

Before this change the :add ghci command documentation in user guide would render like this in my browser.

This makes it look as if the * could be optionally added right after the :add command. But when I run :add* SomeModule in GHCi, I get an error unknown command ':add*'. Turns out the correct way is to add * right in front of the module name(s) you want to add. Even GHCi's help tells me so:

:add [*]<module> ... add module(s) to the current target set