Fix a grammatical error reported by Detlef Lannert
<lannert@lannert.rz.uni-duesseldorf.de>.
diff --git a/Doc/lib/libthreading.tex b/Doc/lib/libthreading.tex
index b9f4e05..eb0985f 100644
--- a/Doc/lib/libthreading.tex
+++ b/Doc/lib/libthreading.tex
@@ -369,7 +369,7 @@
\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