[1.3.X] Refs #16072 -- Corrected blocktrans multiple-argument syntax example in the docs.

You can use multiple expressions inside a single ``blocktrans`` tag::

    {% blocktrans with book_t=book|title and author_t=author|title %}

    {% blocktrans with book_t=book|title author_t=author|title %}

    This is {{ book_t }} by {{ author_t }}

    {% endblocktrans %}