Fix docs about heat in devtack

In 279cfe75198c723519f1fb361b2bff3c641c6cef, we disabled heat by default.
So fixing the README to reflect that.

Change-Id: I3da257158b37e235eed1c78e0c4df432caeefa1d
diff --git a/README.md b/README.md
index 53de970..039ce3e 100644
--- a/README.md
+++ b/README.md
@@ -264,10 +264,10 @@
 
 # Heat
 
-Heat is enabled by default (see `stackrc` file). To disable it explicitly
+Heat is disabled by default (see `stackrc` file). To enable it explicitly
 you'll need the following settings in your `localrc` section:
 
-    disable_service heat h-api h-api-cfn h-api-cw h-eng
+    enable_service heat h-api h-api-cfn h-api-cw h-eng
 
 Heat can also run in standalone mode, and be configured to orchestrate
 on an external OpenStack cloud. To launch only Heat in standalone mode