Fix errors in diamond_iso shader

[rparrett]

Fixes #448

These lines were included in the initial 0.11 update PR, but disappeared afterwards in what I assume was a merge conflict oopsie.

[StarArawn]

Missed this thank you!

[n4zim]

Just for the record @StarArawn, the published crate does not contain this update.

I used [replace] in my Cargo.toml to override and retrieve those changes from the repository but maybe you could publish version 0.11.1 ?

(by the way, thank you for your work with this bevy plugin)