Update readme.md to fix bug w/ retrieve (#62)

The example to get an action doesn't work - this change corrects it.
diff --git a/README.md b/README.md
index 36b4a19..27f3740 100644
--- a/README.md
+++ b/README.md
@@ -146,7 +146,7 @@
 
 ```
 const name = 'reverseWords'
-ow.actions.retrieve({name}).then(action => {
+ow.actions.get(name).then(action => {
   console.log('action resource', action)
 }).catch(err => {
   console.error('failed to retrieve action', err)