Update tutorial.rst (#3394)

eliminated inconsitency of variable name in sample code
diff --git a/docs/x509/tutorial.rst b/docs/x509/tutorial.rst
index 143cdfc..372d78a 100644
--- a/docs/x509/tutorial.rst
+++ b/docs/x509/tutorial.rst
@@ -133,7 +133,7 @@
     ... ).issuer_name(
     ...     issuer
     ... ).public_key(
-    ...     private_key.public_key()
+    ...     key.public_key()
     ... ).serial_number(
     ...     x509.random_serial_number()
     ... ).not_valid_before(
@@ -145,7 +145,7 @@
     ...     x509.SubjectAlternativeName([x509.DNSName(u"localhost")]),
     ...     critical=False,
     ... # Sign our certificate with our private key
-    ... ).sign(private_key, hashes.SHA256(), default_backend())
+    ... ).sign(key, hashes.SHA256(), default_backend())
     >>> # Write our certificate out to disk.
     >>> with open("path/to/certificate.pem", "wb") as f:
     ...     f.write(cert.public_bytes(serialization.Encoding.PEM))