config: Replace em dash — by -- for switch in comment

Fixes: d88d27d ("drop enabled config variables from config file and use defaults")

config

@@ -82,7 +82,7 @@
 
 # Set target architecture.
 # Only useful when installing i386 while running an amd64 kernel.
-# Default: current architecture. (From 'dpkg —print-architecture'.)
+# Default: current architecture. (From 'dpkg --print-architecture'.)
 # ARCH='amd64'
 
 # Hostname of new system.