minor correction for run-single-test

git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@268918 13f79535-47bb-0310-9956-ffa450edef68

docs/ant_task_guidelines.html

@@ -124,7 +124,7 @@ The test cases are a great way of testing your task during development.
 A simple call to 'ant run-test' in the ant source tree will run all ant
 tests, to verify that your changes don't break anything.
 To test a single task, use the one shot <code>ant run-single-test
-${testname}</code> where ${testname} is the name of your test class.
+-Dtestcase=${testname}</code> where ${testname} is the name of your test class.
 
 
 <p>