fixed spelling error in comment

dlib/optimization/optimization.h

@@ -427,7 +427,7 @@ namespace dlib
         for (long i = 0; i < gradient.size(); ++i)
         {
             const double tol = eps*std::abs(x(i));
-            // if x(i) is an active bound constraint then we should set it's search
+            // If x(i) is an active bound constraint then we should set its search
             // direction such that a single step along the direction either does nothing or
             // closes the gap of size tol before hitting the bound exactly.
             if (x_lower(i)+tol >= x(i) && gradient(i) > 0)