doc: Document /bin/sh as default shell

The default shell was changed from /bin/bash to /bin/sh in commit
61a7846c ("config: Use /bin/sh as default shell and don't check for its
existence", 2015-04-05), update the documentation about this.

doc/ncdu.pod

@@ -217,7 +217,7 @@ Spawn shell in current directory.
 We first check the $NCDU_SHELL environment variable of the user for the
 preferred shell interpreter. If it's not set, $SHELL is checked. If that
 variable also isn't defined, we fall back to the compile time configured
-default shell (usually /bin/bash). This allows you to specify which program to
+default shell (usually /bin/sh). This allows you to specify which program to
 launch when 'b' is pressed. For example, if you want ncdu to execute the
 Midnight Commander file manager in the current directory: