Docs: use float in bpy.app.timer examples

Makes it clear seconds are not integers,
also avoids conversion to float.

doc/python_api/examples/bpy.app.timers.2.py

@@ -6,6 +6,6 @@ import bpy
 
 def every_2_seconds():
     print("Hello World")
-    return 2
+    return 2.0
 
 bpy.app.timers.register(every_2_seconds)