Fix a malformed sentence in the tutorial.

Thanks to abrock for pointing this out.

Change-Id: I3fe943dc415dc389396e13dbdd5aec09ee9b76cc

docs/source/nnls_tutorial.rst

diff --git a/docs/source/nnls_tutorial.rst b/docs/source/nnls_tutorial.rst
index 322babd..1ff2096 100644
--- a/docs/source/nnls_tutorial.rst
+++ b/docs/source/nnls_tutorial.rst
@@ -707,8 +707,8 @@
 similar to the curve fitting example -- one term is added to the
 objective function per observation.
 
-Since this large sparse problem (well large for ``DENSE_QR`` anyways),
-one way to solve this problem is to set
+Since this is a large sparse problem (well large for ``DENSE_QR``
+anyways), one way to solve this problem is to set
 :member:`Solver::Options::linear_solver_type` to
 ``SPARSE_NORMAL_CHOLESKY`` and call :member:`Solve`. And while this is
 a reasonable thing to do, bundle adjustment problems have a special