BPY Docs: Correct spelling (Addon --> Add-on)

Differential Revision: https://developer.blender.org/D2293

doc/python_api/examples/bpy.app.handlers.1.py

@@ -4,7 +4,7 @@ Persistent Handler Example
 
 By default handlers are freed when loading new files, in some cases you may
 wan't the handler stay running across multiple files (when the handler is
-part of an addon for example).
+part of an add-on for example).
 
 For this the :data:`bpy.app.handlers.persistent` decorator needs to be used.
 """