Update elp_development.md to improve declarative test example

Summary: We now include the diagnostic code in the expected diagnostic.

Reviewed By: RobinMorisset

Differential Revision: D86658752

fbshipit-source-id: 43f835633a0c81c15d7f43cfcdfde967db9a3178

Author: alanz

elp_development.md

@@ -155,9 +155,8 @@ let fixture = r#"
 //- /src/main.erl
 -module(main).
 
-foo(X) ->
-    X + undefined.
-  %%    ^^^^^^^^^ error: type mismatch
+foo( -> ok. %%
+%%  ^ error: W0004: Missing ')'~
 "#;
 ```