add one exercise in patterns.md

solutions/pattern-match/patterns.md

@@ -85,4 +85,17 @@ fn main() {
         }
     }
 }
+```
+
+6.
+```rust
+fn main() {
+    let mut v = String::from("hello,");
+    let r = &mut v;
+
+    match r {
+        // The type of value is &mut String
+       value => value.push_str(" world!") 
+    }
+}
 ```