Fix issue with Postgres 10 (#138)

* Fixed issue with Postgres 10

When using Postgres 10 the `information_schema.columns.column_name` needs to be cast as text. Otherwise, an error is generated which states:

```
could not find array type for data type information_schema.sql_identifier
```

* Fixed typo

There was a missing space between the -m and venv that could lead to confusion for new contributors

Author: ryancheley

django_sql_dashboard/views.py

@@ -159,7 +159,7 @@ def _dashboard_index(
             )
             select
               information_schema.columns.table_name,
-              array_to_json(array_agg(column_name order by ordinal_position)) as columns
+              array_to_json(array_agg(cast(column_name as text) order by ordinal_position)) as columns
             from
               information_schema.columns
             join

docs/contributing.md

@@ -3,7 +3,7 @@
 To contribute to this library, first checkout the code. Then create a new virtual environment:
 
     cd django-sql-dashboard
-    python -mvenv venv
+    python -m venv venv
     source venv/bin/activate
 
 Or if you are using `pipenv`: