docs: fixing elvish install instructions

[ejrichards]

Previous instructions were not working on the latest version of Elvish, giving the following error:

Exception: wrong type: need ns, got map

I changed it so the namespace is updated in the eval callback.

I also changed the alias to use edit:add-var since the fn syntax is contained to scope, so if the previous code was in, for example, an if statement, it would not export the mise alias.