Fix a typo in small_blas.h

The reason this rather serious looking typo has not
caused any problems uptil now is because NUM_ROW_B is
computed but never actually used.

Thanks to Werner Trobin for pointing this out.

Change-Id: Id2b4d9326ec21baec8a85423e3270aefbafb611e

internal/ceres/small_blas.h

diff --git a/internal/ceres/small_blas.h b/internal/ceres/small_blas.h
index 0da1e7a..264ac53 100644
--- a/internal/ceres/small_blas.h
+++ b/internal/ceres/small_blas.h
@@ -73,7 +73,7 @@
   DCHECK((kColB == Eigen::Dynamic) || (kColB == num_col_b));            \
   const int NUM_ROW_A = (kRowA != Eigen::Dynamic ? kRowA : num_row_a);  \
   const int NUM_COL_A = (kColA != Eigen::Dynamic ? kColA : num_col_a);  \
-  const int NUM_ROW_B = (kColB != Eigen::Dynamic ? kRowB : num_row_b);  \
+  const int NUM_ROW_B = (kRowB != Eigen::Dynamic ? kRowB : num_row_b);  \
   const int NUM_COL_B = (kColB != Eigen::Dynamic ? kColB : num_col_b);
 
 #define CERES_GEMM_EIGEN_HEADER                                         \