Update src/doc/unstable-book/src/compiler-flags/codegen-backend.md

Author: XAMPPRocky

src/doc/unstable-book/src/compiler-flags/codegen-backend.md

@@ -12,7 +12,7 @@ backend. The library must contain a function named `__rustc_codegen_backend`
 with a signature of `fn() -> Box<dyn rustc_codegen_ssa::traits::CodegenBackend>`.
 
 ## Example
-```rust
+```rust,ignore
 use rustc_codegen_ssa::traits::CodegenBackend;
 
 struct MyBackend;