Update newrelic.yml rake task to handle code fences (```)

[kaylareopelle]

The task that generates our newrelic.yml file is not able to handle code fences (```).

An example of this behavior can be seen with the automatic_custom_instrumentation_method_list entry in the 9.14.0 prerelease.

Update the task to handle this markdown feature.

[workato-integration]

https://new-relic.atlassian.net/browse/NR-317359