cmPolicies: Fix typo

Use American spelling: behaviour -> behavior.

Help/policy/CMP0017.rst

@@ -11,7 +11,7 @@ the modules belonging to CMake always get those files included which
 they expect, and against which they were developed and tested.  In all
 other cases, the files found in CMAKE_MODULE_PATH still take
 precedence over the ones in the CMake module directory.  The OLD
-behaviour is to always prefer files from CMAKE_MODULE_PATH over files
+behavior is to always prefer files from CMAKE_MODULE_PATH over files
 from the CMake modules directory.
 
 This policy was introduced in CMake version 2.8.4.  CMake version