Fixed typo in HashMap documentation
Fixed typo in HashMap documentation

Previously "with a custom type as key", now "with a custom key type"
aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust