Added punctuation

Author: yankev

plotly/session.py

@@ -77,9 +77,9 @@ def sign_in(username, api_key, **kwargs):
     for key in kwargs:
         if key not in CREDENTIALS_KEYS and key not in CONFIG_KEYS:
             raise exceptions.PlotlyError("{} is not a valid config or "
-                                         "credentials key\n"
+                                         "credentials key.\n"
                                          "Type `help(sign_in)` for a full list "
-                                         "of sign in related keys".format(key))
+                                         "of sign in related keys.".format(key))
 
     # add credentials, raise error if type is wrong.
     for key in CREDENTIALS_KEYS: