Update E0384.md

Add an example for the shadowing usage.

Author: JimmyOhn

Diff for: compiler/rustc_error_codes/src/error_codes/E0384.md

@@ -18,3 +18,13 @@ fn main() {
     x = 5;
 }
 ```
+
+Otherwise you can use the shadowing instead. declared variable will be destroyed
+when declaring the same variable. For example:
+
+```
+fn main() {
+    let x = 3;
+    let x = 5;
+}
+```