Docs: Fix several errors of examples from the doc (#592)

* [Doc] Fix several errors of examples from the doc

* Add missing operator def.

* Added missing `()`

* Add missing `namespace`.
diff --git a/docs/advanced/cast/stl.rst b/docs/advanced/cast/stl.rst
index bbd2373..16ac0e6 100644
--- a/docs/advanced/cast/stl.rst
+++ b/docs/advanced/cast/stl.rst
@@ -72,7 +72,7 @@
     /* ... binding code ... */
 
     py::class_<MyClass>(m, "MyClass")
-        .def(py::init<>)
+        .def(py::init<>())
         .def_readwrite("contents", &MyClass::contents);
 
 In this case, properties can be read and written in their entirety. However, an