fix typo on package command (#734)

This commit fixes the typo in the package command on the README.md.

README.md

@@ -17,7 +17,7 @@ In Linux operating systems that you can not run docker as a root user you need t
 Running Elastic Agent in a docker container is a common use case. To build the Elastic Agent and create a docker image run the following command:
 
 ```
-DEV=true SNAPSHOT=true PLATFORMS=linux/amd64 TYPES=docker mage package
+DEV=true SNAPSHOT=true PLATFORMS=linux/amd64 PACKAGES=docker mage package
 ```
 
 If you are in the 7.13 branch, this will create the `docker.elastic.co/beats/elastic-agent:7.13.0-SNAPSHOT` image in your local environment. Now you can use this to for example test this container with the stack in elastic-package:
@@ -45,7 +45,7 @@ for the standard variant.
 
 1. Build elastic-agent:
 ```bash
-DEV=true PLATFORMS=linux/amd64 TYPES=docker mage package
+DEV=true PLATFORMS=linux/amd64 PACKAGES=docker mage package
 ```
 
 Use environmental variables `GOHOSTOS` and `GOHOSTARCH` to specify PLATFORMS variable accordingly. eg.