docs: sign Digest header in hmac-auth body validation example

[a27851305-hue]

Description

The body-validation example sends a Digest header, but the documented HMAC signing string and signed-header list did not include it. This updates only that example so the request body digest is bound to the signature and refreshes the matching static outputs.

Validation:

• Recalculated the documented HMAC-SHA256 signature for the sample request.
• Reviewed the rendered one-file diff and confirmed unrelated examples remain unchanged.

Which issue(s) this PR fixes:

Fixes #13395

Checklist

• 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 (documentation-only; the example digest and signature were recalculated)
• 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)