docs: add plugin filter usage example

[soulbird]

Description

Checklist

• [x] I have explained the need for this PR and the problem it solves
• [ ] I have explained the changes or the new features added to this PR
• [ ] 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)

[gxthrj]

LGTM