Update design doc to match current devel branch.

wiki/src/contribute/design.mdwn

@@ -899,7 +899,7 @@ live-boot's `swapon` option is not set.
 Tails takes care not to use any filesystem that might exist on
 the host machine hard drive, unless explicitly told to do so by the
 user. The Debian Live persistence feature is disabled by passing
-`nopersistent` over the kernel command line to live-boot.
+`nopersistence` over the kernel command line to live-boot.
 
 - [[!tails_gitweb config/amnesia]]