Remove non-existent argument from "HelloWorld" example?

[nusbaume]

When using the CPF_0.2.021 tag, which is the same tag that is currently used in CAMDEN, the "HelloWorld" example README.md document contains a reference to a --generate-host-cap argument for ccpp_capgen.py which no longer appears to exist, and which will cause that example to fail. This missing argument will also cause building with the Makefile.example file to fail as well. However, if the argument is left out then ccpp_capgen.py appears to work without any problems.

I am not sure if this argument exists for other ccpp-framework versions, but if this particular branch/fork is the one that will be moving forward then I would recommend removing the --generate-host-cap argument from "HelloWorld", just to avoid any confusion for new users.