Fix typo.

git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@276417 13f79535-47bb-0310-9956-ffa450edef68

src/main/org/apache/tools/ant/taskdefs/Execute.java

@@ -431,7 +431,7 @@ public class Execute {
         }
 
         if (dir != null && !dir.exists()) {
-            throw new BuildException(dir + " doesn't exists.");
+            throw new BuildException(dir + " doesn't exist.");
         }
         return launcher.exec(project, command, env, dir);
     }