How to use VARIABLES for body content?

Hello, I tried using a variable to update a section in a README file but it was returned as is.
```
- name: Update section in readme
run:
SECTION_DETAILS="This is a test content"
uses: jaywcjlove/github-action-modify-file-content@main
with:
path: README.md
body: "$SECTION_DETAILS"
openDelimiter: ""
closeDelimiter: ""
```

The file is updated but this is what I see:
```

$SECTION_DETAILS

```

How can I exactly update the file using variables?

Start with the workflow snippet in the issue and the README.md section it updates. Trace how the action receives the body value and determine the expected variable-expansion behavior; done means documenting or reproducing the exact configuration needed to replace the delimited section with the variable's content.

Written by the indexing model from the issue text.