chore: fix struct name (#2379)

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## Overview

fix struct name

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<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit

- **Documentation**
- Corrected a typo in the comment describing the SignatureVerifier type.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Signed-off-by: xiaoxiangirl <caojiaqiao@outlook.com>

Author: xiaoxiangirl

types/signed_header.go

@@ -37,7 +37,7 @@ var (
 
 var _ header.Header[*SignedHeader] = &SignedHeader{}
 
-// SignatureVerified is a custom signature verifiers.
+// SignatureVerifier is a custom signature verifiers.
 // If set, ValidateBasic will use this function to verify the signature.
 type SignatureVerifier func(header *Header) ([]byte, error)