docs(forms): fix typo

Author: ryang-sfdc

Diff for: docs/content/guide/forms.ngdoc

@@ -113,7 +113,7 @@ This ensures that the user is not distracted with an error until after interacti
 
 # Binding to form and control state
 
-A form is in instance of {@link api/ng.directive:form.FormController FormController}.
+A form is an instance of {@link api/ng.directive:form.FormController FormController}.
 The form instance can optionally be published into the scope using the `name` attribute.
 Similarly control is an instance of {@link api/ng.directive:ngModel.NgModelController NgModelController}.
 The control instance can similarly be published into the form instance using the `name` attribute.