autoreplace ' s '

I can't tell you how many times my iphone will think I'm typing 's' when I mean 'a' in a sentence resulting in something like : 'I need s drink'

I don't think anyone ever uses 's' like that.

I cannot figure out a keyboard replacement shortcut for ' s ' to ' a '. Something about "the shortcut cannot contain spaces"

aside from being more careful when i type 'a', is there any other fix for this?