Fix typo in manpage.

man/ninjahelper.1

@@ -33,7 +33,7 @@ menu-driven interface.
 To add an additional 'wizard' to ninjahelper, follow these steps:
 .IP (1)
 create a file in the handlers directory (eg. /usr/share/backupninja) using
-the .helper extention. For example, if you wish to create a helper for the
+the .helper extension. For example, if you wish to create a helper for the
 handler "blue", create the file /usr/share/backupninja/blue.helper.
 .IP (2)
 next, add your helper to the global HELPERS variable and define the main