docs: fix typo in README.md (#25)

Author: kdubuc

README.md

@@ -63,7 +63,7 @@ const getClientCredentials = oauth.clientCredentials(
  'https://oauth.com/2.0/token',
  'CLIENT_ID',
  'CLIENT_SECRET'
-})
+)
 
 const auth = await getClientCredentials('OPTIONAL_SCOPES')
 // => { "access_token": "...", "expires_in": 900, ... }