Update Branch.cs

Fix documentation typo

Author: yahavx

LibGit2Sharp/Branch.cs

@@ -162,7 +162,7 @@ public virtual string UpstreamBranchCanonicalName
         ///   If this is a local branch, this will return the configured
         ///   <see cref="Remote"/> to fetch from and push to. If this is a
         ///   remote-tracking branch, this will return the name of the remote 
-        ///   containing the tracked branch. If there no tracking information 
+        ///   containing the tracked branch. If there is no tracking information, 
         ///   this will return null.
         /// </para>
         /// </summary>