Fixed typo in cfengine --help

[larsewi]

Before:

$ cfengine --help | grep -A1 'Build a policy set from a'
    build               Build a policy set from a CFEngine Build project A
                        wrapper arount the cfbs `build`-function

After:

$ cfengine --help | grep -A1 'Build a policy set from a'
    build               Build a policy set from a CFEngine Build project. A
                        wrapper around the cfbs `build`-function.