Minor fix to colon

Mostly not worth a PR but I wanted to change it either way. Changed the display output from `::` to `:` as it is in the rest of the document.

Author: wridgeu

routing.rst

@@ -1874,7 +1874,7 @@ use the ``generateUrl()`` helper::
 
  If you pass to the ``generateUrl()`` method some parameters that are not
  part of the route definition, they are included in the generated URL as a
- query string:::
+ query string::
 
  $this->generateUrl('blog', ['page' => 2, 'category' => 'Symfony']);
  // the 'blog' route only defines the 'page' parameter; the generated URL is: