Fix a typo in problem.h

Thanks as usual to William Rucklidge.

Change-Id: If6e8628841ee7fa8978ec56918a80d60b4ff660e

include/ceres/problem.h

diff --git a/include/ceres/problem.h b/include/ceres/problem.h
index 707a8eb..33394ce 100644
--- a/include/ceres/problem.h
+++ b/include/ceres/problem.h
@@ -333,7 +333,7 @@
 
   // The size of local parameterization for the parameter block. If
   // there is no local parameterization associated with this parameter
-  // block, then ParmeterBlockLocalSize = ParameterBlockSize.
+  // block, then ParameterBlockLocalSize = ParameterBlockSize.
   int ParameterBlockLocalSize(double* values) const;
 
   // Fills the passed parameter_blocks vector with pointers to the