PROFILES.md - be really explicit about overriding

While "Declaring Profiles" states that "Profiles specified in `profiles.clj` will override
profiles in `project.clj`," some less attentive reader can interpret it so that *keys/values* from profiles.clj override those from project.clj, not that the profile as whole is replaced. This changes
tries to make it really clear. (Of course feel free to change the wording as you see fit.)
Update PROFILES.md
Merge pull request #1506 from jakubholynet/patch-1

PROFILES.md - be really explicit about overriding