Replaces the double-loop in step-3 results extension with just one loop

Replaces the double-loop in step-3 results extension with just one loop

In step-3, replaces make_grid_and_dofs with just make_grid

In Step-3, removes reference to .gnuplot in subsection title in extensions (results)

In step-3, minor wording change to convey that the suggested boundary condition code is only slightly longer than before

trying to address a comment on indentation by reviewer of PR

adding a blank statement after the 'if' line to address reviewer's clarification

removes the extra line added in prev commit. Also, changes indentation of the loop code to 2 spaces

As per the request of a reviewer, reverts the change made to a sub-heading wherein the .gnuplot was removed