Fix the PR

Fix the grammar/punctuation on this, please... 
"This can either be through an API that is packaged with the mod." <-- only use "either" if you've got two things you're offering as options, eg "either via an API or through using public classes"
"Addons do not take any code" <-- should be "Addons do not re-use any code from the parent mod"
"besides the API if they use it" <-- Not necessary to re-state, you covered that in the second sentence by saying that an addon can extend the mod through an api. As a side note, addons shouldn't ever need to include the API...they assume that the API is there, because if it's not, they shouldn't run (they're an addon, not a standalone mod).