Update blocks.rst
Update blocks.rst

Docs does not currently include the `rows` kwarg that is available on `TextBlock`.

See Question on slack https://wagtailcms.slack.com/archives/C81FGJR2S/p1635825388225100
rsalmaso/wagtail,thenewguy/wagtail,zerolab/wagtail,rsalmaso/wagtail,zerolab/wagtail,zerolab/wagtail,mixxorz/wagtail,thenewguy/wagtail,zerolab/wagtail,thenewguy/wagtail,jnns/wagtail,thenewguy/wagtail,wagtail/wagtail,mixxorz/wagtail,mixxorz/wagtail,rsalmaso/wagtail,jnns/wagtail,rsalmaso/wagtail,mixxorz/wagtail,wagtail/wagtail,zerolab/wagtail,jnns/wagtail,wagtail/wagtail,mixxorz/wagtail,jnns/wagtail,rsalmaso/wagtail,wagtail/wagtail,wagtail/wagtail,thenewguy/wagtail