Fix typo in NIST url.

Change-Id: I27937dc4bf6f0216d54be5c90d9f950b8ced87af

docs/source/nnls_tutorial.rst

diff --git a/docs/source/nnls_tutorial.rst b/docs/source/nnls_tutorial.rst
index acbb9a1..3c39086 100644
--- a/docs/source/nnls_tutorial.rst
+++ b/docs/source/nnls_tutorial.rst
@@ -773,7 +773,7 @@
 #. `nist.cc
    <https://ceres-solver.googlesource.com/ceres-solver/+/master/examples/nist.cc>`_
    implements and attempts to solves the `NIST
-   <http://www.itl.nist.gov/div898/strd/nls/nls_main.shtm>`_
+   <http://www.itl.nist.gov/div898/strd/nls/nls_main.shtml>`_
    non-linear regression problems.
 
 #. `more_garbow_hillstrom.cc