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/pycpp/object.rst b/docs/advanced/pycpp/object.rst
index 8fc165d..8e737cc 100644
--- a/docs/advanced/pycpp/object.rst
+++ b/docs/advanced/pycpp/object.rst
@@ -57,7 +57,7 @@
 
 .. code-block:: cpp
 
-    using pybind11::literals; // to bring in the `_a` literal
+    using namespace pybind11::literals; // to bring in the `_a` literal
     f(1234, "say"_a="hello", "to"_a=some_instance); // keyword call in C++
 
 Unpacking of ``*args`` and ``**kwargs`` is also possible and can be mixed with