Use "opaques" instead of "opaque types" in reporting

crates/compiler/load/tests/test_reporting.rs

@@ -11054,8 +11054,8 @@ All branches in an `if` must have the same type!
     4│      Recursive := [Infinitely Recursive]
             ^^^^^^^^^
 
-    Recursion in opaques is only allowed if recursion happens behind a
-    tagged union, at least one variant of which is not recursive.
+    Recursion in opaque types is only allowed if recursion happens behind
+    a tagged union, at least one variant of which is not recursive.
     "
     );
 

crates/compiler/types/src/types.rs

@@ -3579,7 +3579,7 @@ impl AliasKind {
     pub fn as_str_plural(&self) -> &'static str {
         match self {
             AliasKind::Structural => "aliases",
-            AliasKind::Opaque => "opaques",
+            AliasKind::Opaque => "opaque types",
         }
     }
 }