Add null value to data types doc in protocols

Author: forstisabella

src/protocols/tracking-plan/create.md

@@ -106,7 +106,7 @@ The Tracking Plan editor is organized as a spreadsheet to help you  add new even
 | Name             | Specify the name of your event or property.                                                                                                                                                                                                                                    |
 | Description      | Enter a description for your event or property. These descriptions are helpful for both engineers instrumenting Segment and consumers of the data.                                                                                                                             |
 | Status           | Specify whether a property is required or optional. You can't require a Track call because Segment is unable to verify when a Track call should be fired.                                                                                                            |
-| Data Type        | Specify the data type of the property. Data type options include `any, array, object, boolean, integer, number, string, Date time`. Note: Date time is required to be in ISO-8601 format                                                                                       |
+| Data Type        | Specify the data type of the property. Data type options include `any, array, object, boolean, integer, number, string, null, Date time`. Note: Date time is required to be in ISO-8601 format                                                                                       |
 | Permitted Values | Enter simple regular expressions to validate property values. This works when a property data type is set to `string`. For example, you can add pipe delimited strings to the regex column to generate violations when a property value does not match fall, winter or spring. |
 
 > info ""