POLAK_RIBIRERE -> POLAK_RIBIERE

Thanks to Vladimir Chalupecky for reporting this.

Change-Id: I2e419415394f5d2be35b825d7c777b01ff31add1

include/ceres/types.h

@@ -247,7 +247,7 @@ enum LineSearchDirectionType {
 // details see Numerical Optimization by Nocedal & Wright.
 enum NonlinearConjugateGradientType {
   FLETCHER_REEVES,
-  POLAK_RIBIRERE,
+  POLAK_RIBIERE,
   HESTENES_STIEFEL,
 };