Minor documentation fix.

Thanks to Satya Mallick.

Change-Id: I556f1c141bf16739d54450351b0f29fd4ea40014

docs/source/tutorial.rst

diff --git a/docs/source/tutorial.rst b/docs/source/tutorial.rst
index f1c2cbe..00c65a9 100644
--- a/docs/source/tutorial.rst
+++ b/docs/source/tutorial.rst
@@ -9,7 +9,7 @@
 ========
 Ceres solves robustified non-linear least squares problems of the form
 
-.. math:: \frac{1}{2}\sum_{i=1} \rho_i\left(\left\|f_i\left(x_{i_1}, ... ,x_{i_k}\right)\right\|^2\right).
+.. math:: \frac{1}{2}\sum_{i} \rho_i\left(\left\|f_i\left(x_{i_1}, ... ,x_{i_k}\right)\right\|^2\right).
    :label: ceresproblem
 
 The expression