Updated docs

Author: oisyn

docs/src/writing-shader-crates.md

@@ -52,11 +52,11 @@ Paste the following into `build.rs`
 ```rust,no_run
 use spirv_builder::{MetadataPrintout, SpirvBuilder};
 
-fn main() {
+fn main() -> Result<(), Box<dyn std::error::Error>> {
     SpirvBuilder::new(shader_crate, target)
         .print_metadata(MetadataPrintout::Full)
-        .build()
-        .unwrap(); 
+        .build()?;
+    Ok(())
 }
 ```