Update README with an absolute url to screenshot

On pypi relative urls aren't resolved

Author: pensivebrian

README.rst

@@ -10,7 +10,7 @@ mssql-cli
 
 We’re excited to introduce `mssql-cli`_, a new and interactive command line query tool for SQL Server. This open source tool works cross-platform and proud to be a part of the `dbcli.org community`_. 
 
-.. image:: screenshots/mssql-cli-autocomplete.gif
+.. image:: https://github.com/dbcli/mssql-cli/tree/master/screenshots/mssql-cli-autocomplete.gif
    :align: center