fixed a typo

[SVN r24846]

index.htm

@@ -85,7 +85,7 @@
                               C++ Standard Library Technical Report</a> as a 
                         step toward becoming part of a future C++ Standard.</p>
                         <p>Although Boost was begun by members of the C++ Standards Committee Library 
-                           Working Group, participation has expanded to include thousands 
+                           Working Group, participation has expanded to include thousands of
                            programmers from the C++ community at large.</p>
                         <h2><a name="Membership">Participation</a></h2>
                         <p>If you are interested in participating in Boost, please join