added unsigned() description

was missing…

laravel/documentation/database/schema.md

@@ -71,6 +71,7 @@ Command  | Description
 `$table->blob('data');`  |  BLOB equivalent to the table
 `->nullable()`  |  Designate that the column allows NULL values
 `->default($value)`  |  Declare a default value for a column
+`->unsigned()`  |  Set INTERGER to UNSIGNED
 
 > **Note:** Laravel's "boolean" type maps to a small integer column on all database systems.