Update the playpen link for code in the rust book to agree with rustdoc's new style
Update the playpen link for code in the rust book to agree with rustdoc's new style

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