Don't need the -e. The tox will use [testenv:default] in tox.ini as default

Signed-off-by: Alvin Wang <alvinwang@msn.com>
Change-Id: I4a615b53bf44afde54850fd8cf65e58a100e626e
diff --git a/README.md b/README.md
index 0204846..b392b3c 100644
--- a/README.md
+++ b/README.md
@@ -179,7 +179,7 @@
 
 * Run tests:
     ```
-    $ tox -e tests
+    $ tox tests
     ```
 
 * How to run individual test: