clarified something about the add teacher field (#393)

Small fix though it would have been nice if that field could accept an
email as well.

Author: jairovelasquez

source/instructors/setupcourses/add-teachers.rst

@@ -7,17 +7,17 @@
 Add or Remove Teachers in a Course
 ==================================
 
-Your course may be taught by more than one teacher so you can add teachers to your course. You can also enable a teacher to be a student in the course to test it. Follow these steps to add a teacher to your Codio course:
+Your course may be taught by more than one teacher, so you can add teachers to your course. You can also enable a teacher to be a student in the course to test it. Follow these steps to add a teacher to your Codio course:
 
 1. From the **Courses** page, select the course to open it.
 2. Click the **Teachers** tab.
-3. Click the **Add Member** button and enter their email address and click **Send Invite**.
-4. If the teacher is not a member in your organization then use the field below.
+3. Click the **Add Teacher** button and enter their Codio username and click **Ok**.
+4. If the teacher is not a member of your organization, or you don't know their Codio user name, use the field below.
 
    .. image:: /img/manage_classes/adding_teachers/search_teacher.png
       :alt: Search Teacher
 
-5: If the teacher is not a member of your organization, you can send them an invitation email. Click the **Add Member** button and enter their email address and click **Send Invite**. An email is sent that includes a link for them to sign up for a Codio account. Upon completion, they are automatically added to your organization and course.
+5. If the teacher is not a member of your organization, you can send them an invitation email. Click the **Add Member** button and enter their email address and click **Send Invite**. An email is sent that includes a link for them to sign up for a Codio account. Upon completion, they are automatically added to your organization and course.
 
    .. image:: /img/manage_classes/adding_teachers/inviteteacher.png
       :alt: Invite Teacher