Add a missing term and remove a superfluous word

Change-Id: I25f40f0bf241302b975e6fc14690aa863c0728b0

docs/source/nnls_covariance.rst

diff --git a/docs/source/nnls_covariance.rst b/docs/source/nnls_covariance.rst
index d3b2273..b91e8a9 100644
--- a/docs/source/nnls_covariance.rst
+++ b/docs/source/nnls_covariance.rst
@@ -25,7 +25,7 @@
 observations :math:`y` is the solution to the non-linear least squares
 problem:
 
-.. math:: x^* = \arg \min_x \|f(x)\|^2
+.. math:: x^* = \arg \min_x \|y - f(x)\|^2
 
 And the covariance of :math:`x^*` is given by
 
@@ -169,7 +169,7 @@
       :member:`Covaraince::Options::sparse_linear_algebra_library_type`
       to ``SUITE_SPARSE``.
 
-      Neither ``SPARSE_QR`` cannot compute the covariance if the
+      ``SPARSE_QR`` cannot compute the covariance if the
       Jacobian is rank deficient.