message: Fix link to SQLAlchemy docs about database URLs format
  'Add database' grid refers to SQLAchemy docs, where database URL format is described.
  Documentation for 1.0 does not exist on which results in 404 when following the current URL.
  The link is fixed taking into account [SQLAlchemy 1.2](https://github.com/apache/incubator-superset/blob/master/requirements.txt#L87) as a project dependency.
diff: @@ -187,7 +187,7 @@ class DatabaseView(SupersetModelView, DeleteMixin, YamlExportMixin): # noqa
  'sqlalchemy_uri': utils.markdown(
  'Refer to the '
  '[SqlAlchemy docs]'
  - '(http://docs.sqlalchemy.org/en/rel_1_0/core/engines.html#'
  + '(http://docs.sqlalchemy.org/en/rel_1_2/core/engines.html#'
  'database-urls) '
  'for more information on how to structure your URI.', True),
  'expose_in_sqllab': _('Expose this DB in SQL Lab'),
  