docs: fix example up so it's closer to real code

docs/examples.rst

@@ -137,6 +137,9 @@ We could instead express the above using Mitogen:
 
 ::
 
+    import shutil, os, subprocess
+    import mitogen
+
     def run(*args):
         return subprocess.check_call(args)
 
@@ -144,6 +147,8 @@ We could instead express the above using Mitogen:
         with open(path, 'rb') as fp:
             return s in fp.read()
 
+    @mitogen.main()
+    def main(router):
         device = '/dev/sdb1'
         mount_point = '/media/Media Volume'