Update README.md

Make the `build-args` description clearer - should be specified as;
```
          build-args: |
            KEY1=FOO
            KEY2=BAR
```

and not like;

```
          build-args: KEY1=FOO,KEY2=BAR
```

Signed-off-by: Joshua Greenhalgh <joshuadouglasgreenhalgh@gmail.com>

README.md

@@ -195,7 +195,7 @@ Following inputs can be used as `step.with` keys
 | `add-hosts`        | List/CSV | List of [customs host-to-IP mapping](https://docs.docker.com/engine/reference/commandline/build/#add-entries-to-container-hosts-file---add-host) (e.g., `docker:10.180.0.1`)      |
 | `allow`            | List/CSV | List of [extra privileged entitlement](https://docs.docker.com/engine/reference/commandline/buildx_build/#allow) (e.g., `network.host,security.insecure`)                         |
 | `builder`          | String   | Builder instance (see [setup-buildx](https://github.com/docker/setup-buildx-action) action)                                                                                       |
-| `build-args`       | List     | List of [build-time variables](https://docs.docker.com/engine/reference/commandline/buildx_build/#build-arg)                                                                      |
+| `build-args`       | List     | List of [build-time variables](https://docs.docker.com/engine/reference/commandline/buildx_build/#build-arg) (e.g., key=string, GIT_AUTH_TOKEN=mytoken) - syntax as in the example above for `secrets`                                                                     |
 | `build-contexts`   | List     | List of additional [build contexts](https://docs.docker.com/engine/reference/commandline/buildx_build/#build-context) (e.g., `name=path`)                                         |
 | `cache-from`       | List     | List of [external cache sources](https://docs.docker.com/engine/reference/commandline/buildx_build/#cache-from) (e.g., `type=local,src=path/to/dir`)                              |
 | `cache-to`         | List     | List of [cache export destinations](https://docs.docker.com/engine/reference/commandline/buildx_build/#cache-to) (e.g., `type=local,dest=path/to/dir`)                            |