Fix namespace

Author: julien-michot

src/example.cpp

@@ -3,13 +3,15 @@
 
 #include <tinyopt/tinyopt.h>
 
+using namespace tinyopt::nlls;
+
 int main(int, char **argv) {
 
   double x = 1;
   auto loss = [](const auto &x) { return x * x - 2.0; };
 
-  tinyopt::Options options;
-  const auto &out = tinyopt::Optimize(x, loss, options);
+  Options options;
+  const auto &out = Optimize(x, loss, options);
 
   std::cout << "What's √2 already?\n" << x << "\n";
   return 0;