Help: Fix variable reference in TIMEOUT test property docs

Link to 'CTEST_TEST_TIMEOUT', not 'CTEST_TESTING_TIMEOUT'.

Help/prop_test/TIMEOUT.rst

@@ -6,4 +6,4 @@ How many seconds to allow for this test.
 This property if set will limit a test to not take more than the
 specified number of seconds to run.  If it exceeds that the test
 process will be killed and ctest will move to the next test.  This
-setting takes precedence over CTEST_TESTING_TIMEOUT.
+setting takes precedence over :variable:`CTEST_TEST_TIMEOUT`.