Document health check interval in manifest docs (#4077)

Author: evanfarrar

docs/v3/source/includes/resources/manifests/_object.md.erb

@@ -43,6 +43,7 @@ applications:
     disk_quota: 512M
     health-check-http-endpoint: /healthcheck
     health-check-type: http
+    health-check-interval: 5
     health-check-invocation-timeout: 10
     instances: 3
     memory: 500M
@@ -117,6 +118,7 @@ Name | Type | Description
 **command** | _string_ | The command used to start the process; this overrides start commands from [Procfiles](#procfiles) and buildpacks
 **disk_quota** | _string_ | The disk limit for all instances of the web process; <br>this attribute requires a unit of measurement: `B`, `K`, `KB`, `M`, `MB`, `G`, `GB`, `T`, or `TB` in upper case or lower case
 **health-check-http-endpoint** | _string_ | Endpoint called to determine if the app is healthy
+**health-check-interval** | _integer_ | The interval in seconds between health check requests
 **health-check-invocation-timeout** | _integer_ | The timeout in seconds for individual health check requests for http and port health checks
 **health-check-type** | _string_ | Type of health check to perform; `none` is deprecated and an alias to `process`
 **instances** | _integer_ | The number of instances to run