Add title of file to example (#42)

docs/tutorials/write-a-contract.mdx

@@ -51,7 +51,7 @@ only if a feature is enabled, with `export_if = "[feature-name]"`.
 
 Putting those pieces together a simple contract will look like this.
 
-```rust
+```rust title="src/lib.rs"
 #![no_std]
 use soroban_sdk::{contractimpl, vec, Env, Symbol, Vec};