Clean up E0307 explanation

Author: GuillaumeGomez

src/librustc_error_codes/error_codes/E0307.md

@@ -1,5 +1,19 @@
-This error indicates that the `self` parameter in a method has an invalid
-"receiver type".
+The `self` parameter in a method has an invalid "receiver type".
+
+Erroneous code example:
+
+```compile_fail,E0307
+struct Foo;
+struct Bar;
+
+trait Trait {
+    fn foo(&self);
+}
+
+impl Trait for Foo {
+    fn foo(self: &Bar) {}
+}
+```
 
 Methods take a special first parameter, of which there are three variants:
 `self`, `&self`, and `&mut self`. These are syntactic sugar for
@@ -36,7 +50,7 @@ impl Trait for Foo {
 }
 ```
 
-E0307 will be emitted by the compiler when using an invalid receiver type,
+This error will be emitted by the compiler when using an invalid receiver type,
 like in the following example:
 
 ```compile_fail,E0307