Fix a grammatical error reported by Detlef Lannert

<lannert@lannert.rz.uni-duesseldorf.de>.

Doc/lib/libthreading.tex

@@ -369,7 +369,7 @@ than zero again, wake up that thread.
 \subsection{Event Objects \label{event-objects}}
 
 This is one of the simplest mechanisms for communication between
-threads: one thread signals an event and one or more other thread
+threads: one thread signals an event and one or more other threads
 are waiting for it.
 
 An event object manages an internal flag that can be set to true with