en.pl: Update to explicitly mention single quotation mark being invalid

en.pl: Update to explicitly mention single quotation mark being invalid

- As all characters, except for the single quotation mark, are now allowed in the PSK
   with the base64 encoding implemented then the error message in the English Lang file
   has been changed to explicitly mention the single quotation mark rather than characters
   as a generic message.

Fixes: Bug13029
Tested-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Adolf Belka <adolf.belka@ipfire.org>
Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>