URL change.

Switch from direct URL which changes to the DOI which is expected to
be consistent.

Author: zivy

Python/68_Registration_Errors.ipynb

@@ -29,7 +29,7 @@
     "\n",
     "[1] \"[Registration of Head Volume Images Using Implantable Fiducial Markers](https://www.ncbi.nlm.nih.gov/pubmed/9263002)\", C. R. Maurer Jr. et al., *IEEE Trans Med Imaging*, 16(4):447-462, 1997.\n",
     "\n",
-    "[2] \"[Fiducial registration error and target registration error are uncorrelated](http://spie.org/Publications/Proceedings/Paper/10.1117/12.813601)\", J. Michael Fitzpatrick, *SPIE Medical Imaging: Visualization, Image-Guided Procedures, and Modeling*, 7261:1–12, 2009.\n",
+    "[2] \"[Fiducial registration error and target registration error are uncorrelated](https://doi.org/10.1117/12.813601)\", J. Michael Fitzpatrick, *SPIE Medical Imaging: Visualization, Image-Guided Procedures, and Modeling*, 7261:1–12, 2009.\n",
     "\n",
     "[3] \"[Fiducial point placement and the accuracy of point-based, rigid body registration](https://www.ncbi.nlm.nih.gov/pubmed/11322441)\", J. B. West et al., *Neurosurgery*, 48(4):810-816, 2001."
    ]