docs: fix the boolean value in the forward_auth.md file

[Ajay-singh1]

This fixes the issue reported under #11461

Description

Fixes # (issue) #11461

Checklist

• [x] I have explained the need for this PR and the problem it solves
• [x] I have explained the changes or the new features added to this PR
• [x] I have added tests corresponding to this change
• [x] I have updated the documentation to reflect this change
• [ ] I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)