fix client credentials example

Author: pcarleton

specification/draft/oauth-client-credentials.mdx

@@ -119,11 +119,10 @@ POST /token HTTP/1.1
 Host: auth.example.com
 Content-Type: application/x-www-form-urlencoded
 
-grant_type=client_credentials
-&client_assertion_type=urn%3Aietf%3Aparams%3Aoauth%3Aclient-assertion-type%3Ajwt-bearer
-&client_assertion=eyJhbGciOiJSUzI1NiIsImtpZCI6IjIyIn0.
-     eyJpc3Mi[...omitted for brevity...].
-     cC4hiUPo[...omitted for brevity...]
+grant_type=urn%3Aietf%3Aparams%3Aoauth%3Agrant-type%3Ajwt-bearer
+&assertion=eyJhbGciOiJFUzI1NiIsImtpZCI6IjE2In0.
+    eyJpc3Mi[...omitted for brevity...].
+    J9l-ZhwP[...omitted for brevity...]
 &resource=https%3A%2F%2Fmcp.example.com
 &scope=mcp%3Aread
 ```