Fix incorrect identifier in comment in rust-toolchain.toml.

Author: nnethercote

rust-toolchain.toml

@@ -3,5 +3,5 @@ channel = "nightly-2025-06-23"
 components = ["rust-src", "rustc-dev", "llvm-tools"]
 # commit_hash = be19eda0dc4c22c5cf5f1b48fd163acf9bd4b0a6
 
-# Whenever changing the nightly channel, update the commit hash above, and make
-# sure to change `REQUIRED_TOOLCHAIN` in `crates/rustc_codegen_spirv/build.rs` also.
+# Whenever changing the nightly channel, update the commit hash above, and
+# change `REQUIRED_RUST_TOOLCHAIN` in `crates/rustc_codegen_spirv/build.rs` too.