Improve --profile description

The description of the --profile option talks about the "update" operation.
This is probably meant for operations such as "nix profile install", but the
same option is reused in other subcommands, which do not update the profile,
such as "nix profile {list,history,diff-closures}".

We update the description to make sense in both contexts.

src/libcmd/command.cc

@@ -226,7 +226,7 @@ MixProfile::MixProfile()
 {
     addFlag({
         .longName = "profile",
-        .description = "The profile to update.",
+        .description = "The profile to operate on.",
         .labels = {"path"},
         .handler = {&profile},
         .completer = completePath