Make slice::split_at_mut example demonstrate mutability
Make slice::split_at_mut example demonstrate mutability

Moved the examples from split_at_mut to split_at so the example at
split_at_mut can just demonstrate mutability.

graydon/rust,aidancully/rust,GBGamer/rust,GBGamer/rust,aidancully/rust,graydon/rust,GBGamer/rust,graydon/rust,GBGamer/rust,graydon/rust,aidancully/rust,aidancully/rust,GBGamer/rust,aidancully/rust,aidancully/rust,graydon/rust,GBGamer/rust,GBGamer/rust,GBGamer/rust,graydon/rust