Make const_evaluatable_unchecked lint example not depend on the archi…

…tecture pointer size.

compiler/rustc_session/src/lint/builtin.rs

@@ -2525,7 +2525,7 @@ declare_lint! {
     /// ```rust
     /// const fn foo<T>() -> usize {
     ///     if std::mem::size_of::<*mut T>() < 8 { // size of *mut T does not depend on T
-    ///         std::mem::size_of::<T>()
+    ///         4
     ///     } else {
     ///         8
     ///     }