Modified the Regex 

This modification fixed an error being caused due to the `ciconia` library. Not sure if this is an issue in phosphorum or ciconia. The error was as follows:

>Warning: preg_replace_callback(): Unknown modifier ' ' in F:\xampp\htdocs\test.loc\phosphorum\forum\vendor\kzykhys\ciconia\src\Ciconia\Common\Text.php on line 177  
>Call Stack  
>#	Time	Memory	Function	Location  
>1	0.0010	157592	{main}( )	..\index.php:0  
>2	0.0160	345968	handle ( )	..\index.php:58  
>3	0.0960	775472	Phalcon\Mvc\View\Engine\Volt->render( )	..\index.php:0  
>4	0.1760	1284728	Ciconia\Ciconia->render( )	..\f__xampp_htdocs_test.loc_phosphorum_forum_app_views_discussions_view.volt.php:57  
>5	0.2090	1511224	Ciconia\Markdown->emit( )	..\Ciconia.php:68  
>6	0.2120	1511664	call_user_func_array ( )	..\EmitterTrait.php:64  
>7	0.2120	1511680	Phosphorum\Markdown\BlockQuoteExtension->processBlockQuote( )	..\EmitterTrait.php:64  
>8	0.2120	1512256	Ciconia\Common\Text->replace( )	..\BlockQuoteExtension.php:70  
>9	0.2120	1512800	preg_replace_callback ( )	..\Text.php:177