aws/signer/v4: Fix typo in signer's doc(#2390)
aws/signer/v4: Fix typo in signer's doc(#2390)

Fixes typo in v4 signer's documentation.
aws/aws-sdk-go,yxd-hde/aws-sdk-go,yxd-hde/aws-sdk-go,yxd-hde/aws-sdk-go,yxd-hde/aws-sdk-go,jasdel/aws-sdk-go