Update the definition of "marker trait"

The consensus has been that inheriting from a trait which has associated items does not introduce coherence violations for the subtrait.

Author: sgrif

0000-allow-overlapping-impls-on-marker-traits.md

@@ -21,8 +21,8 @@ cases.
 # Detailed design
 
 For the purpose of this RFC, the definition of a marker trait is a trait with no
-associated functions, which does not inherit from any other trait. The design
-here is quite straightforward. The following code fails to compile today:
+associated items. The design here is quite straightforward. The following code
+fails to compile today:
 
 ```rust
 trait Marker<A> {}