Fixed typo in docs

[skip ci]
diff --git a/docs/advanced/classes.rst b/docs/advanced/classes.rst
index be4bc2e..c0a9cfa 100644
--- a/docs/advanced/classes.rst
+++ b/docs/advanced/classes.rst
@@ -711,7 +711,7 @@
         int m_extra = 0;
     };
 
-Pickling support in Python is enable by defining the ``__setstate__`` and
+Pickling support in Python is enabled by defining the ``__setstate__`` and
 ``__getstate__`` methods [#f3]_. For pybind11 classes, use ``py::pickle()``
 to bind these two functions: