Fix boto3 resource reference docs

[jonathan343]

Overview

This PR fixes a small bug with boto3 resource documentation generation (addresses #4030).

Specifically, if a boto3 resources has references, there is duplicate text generated in the "References" section as shown in the example below.

Example

Reference: https://boto3.amazonaws.com/v1/documentation/api/1.34.61/reference/services/opsworks/layer/index.html#references

Testing

To test this change, I generated boto3 docs with and without the changes and compared the two. There are 17 changes that are all identical to the one below:
Before

----------
References
----------



References are related resource instances that have a belongs-to relationship.
For more information about references refer to the :ref:`Resources Introduction Guide<references_intro>`.

These are the resource's available references:

.. toctree::
  :maxdepth: 1
  :titlesonly:


These are the resource's available references:

.. toctree::
  :maxdepth: 1
  :titlesonly:

  stack

After

----------
References
----------



References are related resource instances that have a belongs-to relationship.
For more information about references refer to the :ref:`Resources Introduction Guide<references_intro>`.

These are the resource's available references:

.. toctree::
  :maxdepth: 1
  :titlesonly:

  stack