A small fix in enforce_slug_naming.rs.

In #119972 the code should have become `E0123` rather than `0123`. This
fix doesn't affect the outcome because the proc macro errors out before
the type of the code is checked, but the fix makes the test's code
consistent with other similar code elsewhere.

tests/ui-fulldeps/session-diagnostic/enforce_slug_naming.rs

@@ -19,6 +19,6 @@ use rustc_errors::{Applicability, MultiSpan};
 extern crate rustc_session;
 
 #[derive(Diagnostic)]
-#[diag(compiletest_example, code = 0123)]
+#[diag(compiletest_example, code = E0123)]
 //~^ ERROR diagnostic slug and crate name do not match
 struct Hello {}

tests/ui-fulldeps/session-diagnostic/enforce_slug_naming.stderr

@@ -1,7 +1,7 @@
 error: diagnostic slug and crate name do not match
   --> $DIR/enforce_slug_naming.rs:22:8
    |
-LL | #[diag(compiletest_example, code = 0123)]
+LL | #[diag(compiletest_example, code = E0123)]
    |        ^^^^^^^^^^^^^^^^^^^
    |
    = note: slug is `compiletest_example` but the crate name is `rustc_dummy`