Fix typo in BasePGRetriever causing graph context to not be added

[geodavic]

Description

In the _add_source_text() method of BasePGRetriever, there is a mis-named variable that causes the graph content to never be included when creating the source text for a retrieval. This fixes the variable name.

Fixes # (issue)

Version Bump?

Did I bump the version in the pyproject.toml file of the package I am updating? (Except for the llama-index-core package)

• Yes
• No

Type of Change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• I stared at the code and made sure it makes sense
• Tested locally

Suggested Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• I ran make format; make lint to appease the lint gods