Update rustdoc-ui test

tests/rustdoc-ui/track-diagnostics.rs

@@ -11,3 +11,4 @@
 //~^ ERROR mismatched types
 //~| NOTE created at
 //~| NOTE expected `A`, found `B`
+//~| NOTE expected because of the type of the const

tests/rustdoc-ui/track-diagnostics.stderr

@@ -2,7 +2,9 @@ error[E0308]: mismatched types
   --> $DIR/track-diagnostics.rs:LL:CC
    |
 LL | pub const S: A = B;
-   |                  ^ expected `A`, found `B`
+   |              -   ^ expected `A`, found `B`
+   |              |
+   |              expected because of the type of the constant
    |
    = note: -Ztrack-diagnostics: created at compiler/rustc_trait_selection/src/error_reporting/infer/mod.rs:LL:CC