Fix misuse of getopt.

Fix typo of an option name.

multibootusb

@@ -142,7 +142,7 @@ if __name__ == '__main__':
         opts, args = getopt.getopt(
             sys.argv[1:], 'i:t:yvhcudrsp:',
             ['iso=', 'target=', 'yes', 'version', 'help', 'command',
-             'uninstall', 'debug', 'raw', 'syslinux', 'persistence-size'])
+             'uninstall', 'debug', 'raw', 'syslinux', 'persistence-size='])
     except getopt.GetoptError:
         usage()
         sys.exit(2)
@@ -172,7 +172,7 @@ if __name__ == '__main__':
             config.cli_dd = True
         elif opt in ('-s', '--syslinux'):
             config.cli_syslinux = True
-        elif opt in ('-p', '--persisence-size'):
+        elif opt in ('-p', '--persistence-size'):
             config.persistence = int(arg) * 1024 * 1024
         else:
             gui = True