Update example values for plugins

The change I made here #3014 broken the examples. 

This works w/ my original change:
```
controller:
  trafficRouterPlugins:
    - name: "argoproj-labs/gatewayAPI"
      location: "https://github.com/argoproj-labs/rollouts-plugin-trafficrouter-gatewayapi/releases/download/v0.4.0/gatewayapi-plugin-linux-arm64"
      sha256: "07e2fe515c900899c96777aac0ab490437a3d62c8adb94512ccb8b859325eb11"
```

But the example shows stringification is needed.

I feel having it "look like a real thing instead of a string" is more user friendly.

Signed-off-by: Robert Macaulay <[email protected]>

charts/argo-rollouts/values.yaml

@@ -210,14 +210,14 @@ controller:
   # -- Configures 3rd party metric providers for controller
   ## Ref: https://argo-rollouts.readthedocs.io/en/stable/analysis/plugins/
   metricProviderPlugins: {}
-    # metricProviderPlugins: |-
+    # metricProviderPlugins:
     #   - name: "argoproj-labs/sample-prometheus" # name of the plugin, it must match the name required by the plugin so that it can find its configuration
     #     location: "file://./my-custom-plugin" # supports http(s):// urls and file://
 
   # -- Configures 3rd party traffic router plugins for controller
   ## Ref: https://argo-rollouts.readthedocs.io/en/stable/features/traffic-management/plugins/
   trafficRouterPlugins: {}
-    # trafficRouterPlugins: |-
+    # trafficRouterPlugins:
     #   - name: "argoproj-labs/sample-nginx" # name of the plugin, it must match the name required by the plugin so it can find it's configuration
     #     location: "file://./my-custom-plugin" # supports http(s):// urls and file://