Fix some matrix coefficients in the step-25 intro.
Fix some matrix coefficients in the step-25 intro.

The above formula has the correct sign on N. Additionally, the Laplace
matrix here is missing the coefficients.

sairajat/dealii,kalj/dealii,shakirbsm/dealii,EGP-CIG-REU/dealii,JaeryunYim/dealii,shakirbsm/dealii,jperryhouts/dealii,nicolacavallini/dealii,nicolacavallini/dealii,sairajat/dealii,sairajat/dealii,angelrca/dealii,danshapero/dealii,danshapero/dealii,shakirbsm/dealii,naliboff/dealii,JaeryunYim/dealii,naliboff/dealii,spco/dealii,jperryhouts/dealii,pesser/dealii,spco/dealii,kalj/dealii,pesser/dealii,angelrca/dealii,angelrca/dealii,pesser/dealii,spco/dealii,jperryhouts/dealii,JaeryunYim/dealii,naliboff/dealii,nicolacavallini/dealii,jperryhouts/dealii,jperryhouts/dealii,danshapero/dealii,spco/dealii,nicolacavallini/dealii,sairajat/dealii,shakirbsm/dealii,kalj/dealii,JaeryunYim/dealii,EGP-CIG-REU/dealii,JaeryunYim/dealii,spco/dealii,pesser/dealii,angelrca/dealii,JaeryunYim/dealii,angelrca/dealii,danshapero/dealii,kalj/dealii,EGP-CIG-REU/dealii,nicolacavallini/dealii,sairajat/dealii,naliboff/dealii,angelrca/dealii,pesser/dealii,jperryhouts/dealii,danshapero/dealii,EGP-CIG-REU/dealii,jperryhouts/dealii,shakirbsm/dealii,danshapero/dealii,danshapero/dealii,JaeryunYim/dealii,naliboff/dealii,nicolacavallini/dealii,angelrca/dealii,naliboff/dealii,kalj/dealii,shakirbsm/dealii,sairajat/dealii,naliboff/dealii,EGP-CIG-REU/dealii,shakirbsm/dealii,EGP-CIG-REU/dealii,nicolacavallini/dealii,spco/dealii,kalj/dealii,kalj/dealii,sairajat/dealii,spco/dealii,pesser/dealii,EGP-CIG-REU/dealii,pesser/dealii