doc-example: Refactor pointer name to avoid confusion
doc-example: Refactor pointer name to avoid confusion

Changed raw pointer name from ptr to raw_pointer to avoid confusion with
the `use std::ptr` statement a few lines above. This way the crate name
and pointer name are well differenciated.

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