Hotfix 'Attribute error in generate code_challenge ex.' (#1205)

* Hotfix 'Attribute error in generate code_challing ex.'

* Added new authors 'Islam Kamel'

Author: islam-kamel

AUTHORS

@@ -3,6 +3,7 @@ Authors
 
 Massimiliano Pippi
 Federico Frenguelli
+Islam Kamel
 
 Contributors
 ------------

docs/getting_started.rst

@@ -268,7 +268,7 @@ Now let's generate an authentication code grant with PKCE (Proof Key for Code Ex
     code_verifier = ''.join(random.choice(string.ascii_uppercase + string.digits) for _ in range(random.randint(43, 128)))
     code_verifier = base64.urlsafe_b64encode(code_verifier.encode('utf-8'))
 
-    code_challenge = hashlib.sha256(code_verifier.encode('utf-8')).digest()
+    code_challenge = hashlib.sha256(code_verifier).digest()
     code_challenge = base64.urlsafe_b64encode(code_challenge).decode('utf-8').replace('=', '')
 
 Take note of ``code_challenge`` since we will include it in the code flow URL. It should look something like ``XRi41b-5yHtTojvCpXFpsLUnmGFz6xR15c3vpPANAvM``.