fix #379 typos

Author: jrief

docs/source/usage.rst

@@ -61,7 +61,7 @@ Django. Use one of these models fields:
 * :class:`~django.db.models.SmallIntegerField`
 
 In addition to the recommended fields, :class:`~django.db.models.DecimalField` or
-:class:`~django.db.models.FloatField` may work, but haven't been testest.
+:class:`~django.db.models.FloatField` may work, but haven't been tested.
 
 .. warning:: Do not make this field unique! See below why.
 
@@ -70,7 +70,7 @@ In Django's Admin, make the List View sortable
 ==============================================
 
 If a models contains an ordering field, all we need to make the Django's Admin List View sortable,
-is adding the mixin class :class:`adminsortable2.admin.SortableAdminMixin`. By ineriting from this
+is adding the mixin class :class:`adminsortable2.admin.SortableAdminMixin`. By inserting from this
 mixin class together with :class:`~django.contrib.admin.ModelAdmin`, we get a list view which
 without any further configuration, owns the functionality to sort its items.