Clean up E0067 long explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0067.md

@@ -1,33 +1,15 @@
-The left-hand side of a compound assignment expression must be a place
-expression. A place expression represents a memory location and includes
-item paths (ie, namespaced variables), dereferences, indexing expressions,
-and field references.
+An invalid left-hand side expression was used on an assignment operation.
 
-Let's start with some erroneous code examples:
+Erroneous code example:
 
 ```compile_fail,E0067
-use std::collections::LinkedList;
-
-// Bad: assignment to non-place expression
-LinkedList::new() += 1;
-
-// ...
-
-fn some_func(i: &mut i32) {
-    i += 12; // Error : '+=' operation cannot be applied on a reference !
-}
+12 += 1; // error!
 ```
 
-And now some working examples:
+You need to have a place expression to be able to assign it something. For
+example:
 
 ```
-let mut i : i32 = 0;
-
-i += 12; // Good !
-
-// ...
-
-fn some_func(i: &mut i32) {
-    *i += 12; // Good !
-}
+let mut x: i8 = 12;
+x += 1; // ok!
 ```