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   
>0.2120	1511680	Phosphorum\Markdown\BlockQuoteExtension->processBlockQuote( )\EmitterTrait.php:64  
>0.2120	1512256	Ciconia\Common\Text->replace( )	..\BlockQuoteExtension.php:70  
>0.2120	1512800	preg_replace_callback ( )	..\Text.php:177