Update datamodel.md to use an array argument to Enum instead of the more error-prone string

The previous argument to the Enum was: 
"Status" => "Enum('Active, Injured, Retired')"

This would cause issues where the 'Injured' and 'Retired' statuses would actually be stored in the database as ' Injured' and ' Retired' respectively, at least in 3.0.

Using an array() to specify the enum, and using normal array syntax (albeit within a string) is less likely to cause errors.
Merge pull request #2495 from madmatt/patch-1

Update datamodel.md to use an array argument to Enum instead of the more...