Fixup signer header rules excludeList struct/func descriptions (aws#3970

)

Related to the work done in aws#3968 - noted there were a few minor typos made when flipping to allow/exclude named lists.

aws/signer/v4/header_rules.go

@@ -44,12 +44,12 @@ func (w allowList) IsValid(value string) bool {
 	return w.rule.IsValid(value)
 }
 
-// excludeList is a generic rule for blacklisting
+// excludeList is a generic rule for exclude listing
 type excludeList struct {
 	rule
 }
 
-// IsValid for allow list checks if the value is within the allow list
+// IsValid for exclude list checks if the value is within the exclude list
 func (b excludeList) IsValid(value string) bool {
 	return !b.rule.IsValid(value)
 }