Update docs/_docs/reference/other-new-features/opaques-details.md

Co-authored-by: Jamie Thompson <bishbashboshjt@gmail.com>

Author: odersky

docs/_docs/reference/other-new-features/opaques-details.md

@@ -51,7 +51,7 @@ object o:
 def id(x: o.T): o.T = x
 ```
 
-Opaque type aliases cannot be `private` and cannot overridden in subclasses.
+Opaque type aliases cannot be `private` and cannot be overridden in subclasses.
 
 ## Type Parameters of Opaque Types