doc(parser): Rephrase --discover help to avoid confusion

The flag expects a list of python executables, but not paths.
Eg `--discover /foo/bar/python` is good, `--discover /foo/bar` or
`--discover /foo/bar/` won't work.

src/tox/config/cli/parser.py

@@ -233,7 +233,7 @@ def __call__(
             nargs="+",
             metavar="path",
             of_type=list[str],
-            help="for Python discovery first try the Python executables under these paths",
+            help="for Python discovery first try these Python executables",
             default=[],
         )
         list_deps = sub_parser.add_mutually_exclusive_group()