Add missing s

proposals/4037-thread-root-is-not-in-thread.md

@@ -83,7 +83,7 @@ We propose that thread roots are in the main timeline, making the definition:
 >
 > Note: thread roots (events that are referred to in a `m.thread` relationship)
 > are in the main timeline. Similarly, reactions to thread roots, edits of
-> thread root, and other events with non-thread relations to a thread root are
+> thread roots, and other events with non-thread relations to a thread root are
 > in the main timline.
 
 ## Potential issues