doc: string immutability example

doc/docs.md

@@ -355,7 +355,14 @@ println(s) // "hello world"
 
 In V, a string is a read-only array of bytes. String data is encoded using UTF-8.
 
-Strings are immutable.
+Just like in Go and Java, strings are immutable, which means their values cannot be changed.
+
+The following code will raise an error:
+
+```v
+mut s := 'hello'
+s[0] = `H`
+```
 
 Both single and double quotes can be used to denote strings. For consistency,
 `vfmt` converts double quotes to single quotes unless the string contains a single quote character.