Create a link to Tiingo description in ToC (#509)

Tiingo is missing a link in Table of Contents

Author: tiingo

docs/source/remote_data.rst

@@ -29,6 +29,7 @@ extract data from various Internet sources into a pandas DataFrame.
 Currently the following sources are supported:
 
     - :ref:`Google Finance<remote_data.google>`
+    - :ref:`Tiingo<remote_data.tiingo>`
     - :ref:`Morningstar<remote_data.morningstar>`
     - :ref:`IEX<remote_data.iex>`
     - :ref:`Robinhood<remote_data.robinhood>`
@@ -506,4 +507,4 @@ The Moscow Exchange (MOEX) provides historical data.
 
    import pandas_datareader.data as web
    f = web.DataReader('USD000UTSTOM', 'moex', start='2017-07-01', end='2017-07-31')
-   f.head()
+   f.head()