Bless clippy ui tests after format_args change

src/tools/clippy/tests/ui/to_string_in_display.stderr

@@ -6,5 +6,14 @@ LL |         write!(f, "{}", self.to_string())
    |
    = note: `-D clippy::to-string-in-display` implied by `-D warnings`
 
-error: aborting due to previous error
+error: unnecessary use of `to_string`
+  --> $DIR/to_string_in_display.rs:55:50
+   |
+LL |             Self::E(string) => write!(f, "E {}", string.to_string()),
+   |                                                  ^^^^^^^^^^^^^^^^^^
+   |
+   = note: `-D clippy::unnecessary-to-owned` implied by `-D warnings`
+   = note: this error originates in the macro `$crate::format_args` (in Nightly builds, run with -Z macro-backtrace for more info)
+
+error: aborting due to 2 previous errors