Fix Err*NoPub value

[kentakayama]

"PrivateKey" => "PublicKey" because this error can be returned from validate(KeyOpVerify) here:

go-cose/key.go

Lines 657 to 660 in 022cb54

	func (k *Key) PublicKey() (crypto.PublicKey, error) {
	if err := k.validate(KeyOpVerify); err != nil {
	return nil, err
	}

go-cose/key.go

Lines 433 to 441 in 022cb54

	func (k Key) validate(op KeyOp) error {
	switch k.Type {
	case KeyTypeEC2:
	crv, x, y, d := k.EC2()
	switch op {
	case KeyOpVerify:
	if len(x) == 0 || len(y) == 0 {
	return ErrEC2NoPub
	}

[yogeshbdeshpande]

LGTM!

Thanks for the change!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.33%. Comparing base (58542e2) to head (039380d).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #221      +/-   ##
==========================================
+ Coverage   91.23%   92.33%   +1.10%     
==========================================
  Files          13       13              
  Lines        2133     1723     -410     
==========================================
- Hits         1946     1591     -355     
+ Misses        128       73      -55     
  Partials       59       59              

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