fix doc example to submit valid json in POST body

I don't know if there is some version of curl that auto-generates json but the example didn't work for me on curl 7.32.0. Submitting the data as JSON works though.

website/source/docs/auth/userpass.html.md

@@ -35,7 +35,7 @@ The password should be sent in the POST body encoded as JSON.
 
 ```shell
 $ curl $VAULT_ADDR/v1/auth/userpass/login/mitchellh \
-    -d "password=foo"
+    -d '{"password: "foo"}'
 ```
 
 The response will be in JSON. For example: