Harmonising capitalisation of "token" in error strings (#97)

Author: yoogoc

claims.go

@@ -61,7 +61,7 @@ func (c RegisteredClaims) Valid() error {
 	}
 
 	if !c.VerifyIssuedAt(now, false) {
-		vErr.Inner = fmt.Errorf("Token used before issued")
+		vErr.Inner = fmt.Errorf("token used before issued")
 		vErr.Errors |= ValidationErrorIssuedAt
 	}
 
@@ -148,7 +148,7 @@ func (c StandardClaims) Valid() error {
 	}
 
 	if !c.VerifyIssuedAt(now, false) {
-		vErr.Inner = fmt.Errorf("Token used before issued")
+		vErr.Inner = fmt.Errorf("token used before issued")
 		vErr.Errors |= ValidationErrorIssuedAt
 	}