Fix copy in teacher invitation email (#369)

Replacing "create coding lessons for students" with "create coding
projects for students" as per the copy review with Max and Mel.

Author: chrislo

app/views/invitation_mailer/invite_teacher.text.erb

@@ -2,7 +2,7 @@ You have been invited to join:
 
 <%= @school.name %>
 
-Being part of this school account will allow you to access the school dashboard in the Code Editor and create coding lessons for students.
+Being part of this school account will allow you to access the school dashboard in the Code Editor and create coding projects for students.
 
 Join school: