oauth2: fix error message typo

PKSC1 is not a thing, but PKCS1 is, and the parse function above the
error is for PKCS1.

Change-Id: I163bb158070da462caa034a19a89ce654acc7a5d
Reviewed-on: https://go-review.googlesource.com/c/146178
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/internal/oauth2.go b/internal/oauth2.go
index fc63fca..c0ab196 100644
--- a/internal/oauth2.go
+++ b/internal/oauth2.go
@@ -26,7 +26,7 @@
 	if err != nil {
 		parsedKey, err = x509.ParsePKCS1PrivateKey(key)
 		if err != nil {
-			return nil, fmt.Errorf("private key should be a PEM or plain PKSC1 or PKCS8; parse error: %v", err)
+			return nil, fmt.Errorf("private key should be a PEM or plain PKCS1 or PKCS8; parse error: %v", err)
 		}
 	}
 	parsed, ok := parsedKey.(*rsa.PrivateKey)