Change wording to less jaron-y "non-auto trait"

Co-authored-by: NVadim Petrochenkov <vadim.petrochenkov@gmail.com>

compiler/rustc_typeck/src/coherence/inherent_impls.rs

@@ -67,7 +67,7 @@ fn visit_item(&mut self, item: &hir::Item<'_>) {
                     E0785,
                     "cannot define inherent `impl` for a dyn auto trait"
                 )
-                .span_label(ty.span, "impl requires a principal trait")
+                .span_label(ty.span, "impl requires at least one non-auto trait")
                 .note("define and implement a new trait or type instead")
                 .emit();
             }

src/test/ui/coherence/issue-85026.stderr

@@ -2,7 +2,7 @@ error[E0785]: cannot define inherent `impl` for a dyn auto trait
   --> $DIR/issue-85026.rs:5:6
    |
 LL | impl dyn AutoTrait {}
-   |      ^^^^^^^^^^^^^ impl requires a principal trait
+   |      ^^^^^^^^^^^^^ impl requires at least one non-auto trait
    |
    = note: define and implement a new trait or type instead
 
@@ -10,7 +10,7 @@ error[E0785]: cannot define inherent `impl` for a dyn auto trait
   --> $DIR/issue-85026.rs:8:6
    |
 LL | impl dyn Unpin {}
-   |      ^^^^^^^^^ impl requires a principal trait
+   |      ^^^^^^^^^ impl requires at least one non-auto trait
    |
    = note: define and implement a new trait or type instead