Fix Crate version to 3.8 in README instructions
Fix Crate version to 3.8 in README instructions

arrayfire/arrayfire-rust,arrayfire/arrayfire_rust,9prady9/arrayfire-rust,9prady9/arrayfire-rust,arrayfire/arrayfire-rust,9prady9/arrayfire-rust,9prady9/arrayfire-rust,arrayfire/arrayfire-rust