Update the documentation to include ephemeral_storage

Author: tabito-hara

website/docs/r/synthetics_canary.html.markdown

@@ -76,6 +76,7 @@ The following arguments are optional:
 * `memory_in_mb` - (Optional) Maximum amount of memory available to the canary while it is running, in MB. The value you specify must be a multiple of 64.
 * `active_tracing` - (Optional) Whether this canary is to use active AWS X-Ray tracing when it runs. You can enable active tracing only for canaries that use version syn-nodejs-2.0 or later for their canary runtime.
 * `environment_variables` - (Optional) Map of environment variables that are accessible from the canary during execution. Please see [AWS Docs](https://docs.aws.amazon.com/lambda/latest/dg/configuration-envvars.html#configuration-envvars-runtime) for variables reserved for Lambda.
+* `ephemeral_storage` - (Optional) Amount of ephemeral storage (in MB) allocated for the canary run during execution. Defaults to 1024.
 
 ### vpc_config