fix silly typo

Author: mikemccand

help/tests.txt

@@ -139,7 +139,7 @@ Larger heap size
 By default tests run with a 512 MB max heap.  But some tests (monster/nightly)
 need more heap.  Use "-Dtests.heapsize" for this:
 
-  gradlew -p lucene/core test --tests "Test2BFST" -Dtest.heapsize=32g
+  gradlew -p lucene/core test --tests "Test2BFST" -Dtests.heapsize=32g
 
 
 Run GUI tests headlessly with Xvfb (Linux only)