Fix typo in error code description

s/abitrary/arbitrary/

compiler/rustc_error_codes/src/error_codes/E0424.md

@@ -21,7 +21,7 @@ impl Foo {
 The `self` keyword can only be used inside methods, which are associated
 functions (functions defined inside of a `trait` or `impl` block) that have a
 `self` receiver as its first parameter, like `self`, `&self`, `&mut self` or
-`self: &mut Pin<Self>` (this last one is an example of an ["abitrary `self`
+`self: &mut Pin<Self>` (this last one is an example of an ["arbitrary `self`
 type"](https://github.com/rust-lang/rust/issues/44874)).
 
 Check if the associated function's parameter list should have contained a `self`