Minor correction to the README file.

Change-Id: Ic41672692be9d924cebfa8bb4b42b628a49b8bdf

README

diff --git a/README b/README
index 8dd8ccf..14fd355 100644
--- a/README
+++ b/README
@@ -1,3 +1,3 @@
 Ceres Solver - A non-linear least squares minimizer
 ==================================================
-Please see ceres.pdf in docs/ for a tutorial and reference.
+Please see ceres-solver.pdf in docs/ for a tutorial and reference.