Modular Contracts typo fix

Author: maxisch

docs/examples/modular.rst

@@ -7,7 +7,7 @@ Modular Contracts
 A modular approach to building your contracts helps you reduce the complexity
 and improve the readability which will help to identify bugs and vulnerabilities
 during development and code review.
-If you specify and control the behaviour or each module in isolation, the
+If you specify and control the behaviour of each module in isolation, the
 interactions you have to consider are only those between the module specifications
 and not every other moving part of the contract.
 In the example below, the contract uses the ``move`` method