fix undestanding archetypes link

Author: ConnorWelch-OSU

source/user/tutorial/add_reg_inst.rst

@@ -28,7 +28,7 @@ you can run ``cyclus --a`` to check that the archetypes are installed.
 
 Since these are all archetypes, no matter what library they're from, we must include 
 them into the the Archetypes block that we already have.
-Using the template on the `Understanding Archetypes <https://fuelcycle.org/user/tutorial/add_arche.html>`_page,
+Using the template on the `Understanding Archetypes <https://fuelcycle.org/user/tutorial/add_arche.html>`_ page,
 append two ``spec`` blocks into the Archetypes block for the variables listed in the table below.
 
 +-------------+-------------+------------------+