Update docs/types.rst

Co-Authored-By: ChrisChinchilla <chriswhward@gmail.com>

docs/types.rst

@@ -13,7 +13,7 @@ Solidity provides several elementary types which can be combined to form complex
 In addition, types can interact with each other in expressions containing
 operators. For a quick reference of the various operators, see :ref:`order`.
 
-The concept of "undefined" or "null" values do not exist in Solidity, but newly
+The concept of "undefined" or "null" values does not exist in Solidity, but newly
 declared variables always have a :ref:`default value<default-value>` dependent
 on its type. To handle any unexpected values, you should use the :ref:`revert function<assert-and-require>` to revert the whole transaction, or return a
 tuple with a second `bool` value denoting success.