Remove extra space in GROUP_MAX_RECIPIENTS_EXPLAIN

The following string from language\en\acp*\group.php*

contains double space after "private message.":

    'GROUP_MAX_RECIPIENTS_EXPLAIN'    => 'The maximum number of allowed recipients in a private message.  The maximum for all groups of the user is used to determine the actual value.<br />Set this value to 0 to overwrite the setting for all users of this group with the board-wide setting.',

I suggest removing one space so there is only one.