Update diagnostics.rs

Author: GuillaumeGomez

src/librustc_typeck/diagnostics.rs

@@ -171,25 +171,33 @@ function's return type and the value being returned.
 "##,
 
 E0070: r##"
-You tried to change the value of a const variable, which is not possible. Bad
-example:
+The left-hand side of an assignment operator must be an lvalue expression. An
+lvalue expression represents a memory location and includes item paths (ie,
+namespaced variables), dereferences, indexing expressions, and field references.
+
+More details can be found here:
+https://doc.rust-lang.org/reference.html#lvalues,-rvalues-and-temporaries
 
+Now, we can go further. Here are some bad examples:
 ```
 const SOME_CONST : i32 = 12;
 
 fn some_function() {
     SOME_CONST = 14; // error !
+    1=3; // error 
 }
 ```
 
-Constant variables' value cannot be changed once it has been set. Please take a
-look to static keyword if you want something similar but mutable:
+And now let's give good examples:
 
 ```
 static mut SOME_NOT_CONST : i32 = 12;
 
 fn some_function() {
     SOME_NOT_CONST = 14; // that's good !
+    let mut i : i32 = 1;
+    
+    i = 3; // that's good !
 }
 ```
 "##,