Fix `gradient_check_relative_precision` docs typo

In the docs, `Solver::Options::gradient_check_relative_precision` is
documented to have a default precision of 1e08. That is incorrect, it
should be 1e-8.

Change-Id: Ib6b29e356738de4b555c77f7ffd1224d0cae4b6a

docs/source/nnls_solving.rst

diff --git a/docs/source/nnls_solving.rst b/docs/source/nnls_solving.rst
index d03e62e..3399f4d 100644
--- a/docs/source/nnls_solving.rst
+++ b/docs/source/nnls_solving.rst
@@ -1571,7 +1571,7 @@
 
 .. member:: double Solver::Options::gradient_check_relative_precision
 
-   Default: ``1e08``
+   Default: ``1e-8``
 
    Precision to check for in the gradient checker. If the relative
    difference between an element in a Jacobian exceeds this number,