minor adjustment

lecture_6.md

@@ -172,7 +172,7 @@ Sequence of steps (same procedure as in 1D, but harder because of the need to im
 
 The resulting band structure looks like this (in the extended Brillouin zone scheme):
 
-<iframe width="600", height="600" src="figures/nfem_2d.html" frameBorder="0" align="center"></iframe>
+<iframe width="600", height="600" src="figures/nfem_2d.html" frameBorder="0" align="center" style="border:0;">Your browser still doesn't support iframes??</iframe>
 
 Observe that the top of the first band is above the bottom of the lowest band. Therefore if $V$ is sufficiently weak, the material can be conducting even with 2 electrons per unit cell!