Fix typo noticed by Nico Weber

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@44739 91177308-0d34-0410-b5e6-96231b3b80d8

www/index.html

@@ -26,7 +26,7 @@
        motivations for starting work on a new front-end that could
        meet these needs.</p>
        
-    <p>An good (but quite dated) introduction to Clang can be found in the
+    <p>A good (but quite dated) introduction to Clang can be found in the
        following video lectures:</p>
        
     <ul>