Update compiler/rustc_error_codes/src/error_codes/E0384.md

Add an example for the shadowing usage.

compiler/rustc_error_codes/src/error_codes/E0384.md

@@ -18,3 +18,16 @@ fn main() {
     x = 5;
 }
 ```
+
+Alternatively, you might consider initializing a new variable: either with a new
+bound name or (by [shadowing]) with the bound name of your existing variable.
+For example:
+
+[shadowing]: https://doc.rust-lang.org/book/ch03-01-variables-and-mutability.html#shadowing
+
+```
+fn main() {
+    let x = 3;
+    let x = 5;
+}
+```