Update README.txt (#25975)

I found this link is expired. So just update it.

examples/models/nrrd/README.txt

@@ -4,7 +4,7 @@
 ## stent.nrrd
 
 A 3D volume converted from stent.npz from the imageio project.
-https://imageio.readthedocs.io/en/stable/getting_started/standardimages.html
+https://imageio.readthedocs.io/en/stable/user_guide/standardimages.html
 
 It is in the public domain.