scripts: Fix help message in cros_setup_toolchains

The help for the "--nousepkg" flag was inverted. When this flag is used,
it disables the use of prebuilt (binary) packages.

BUG=none
TEST=cros_setup_toolchains -h

Change-Id: Iae299dfb637854913be292772bc1e72fc483676d
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/3693180
Tested-by: Tom Hughes <tomhughes@chromium.org>
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>
Reviewed-by: Jack Neus <jackneus@google.com>
Commit-Queue: Tom Hughes <tomhughes@chromium.org>
diff --git a/scripts/cros_setup_toolchains.py b/scripts/cros_setup_toolchains.py
index 9b009d5..1dda934 100644
--- a/scripts/cros_setup_toolchains.py
+++ b/scripts/cros_setup_toolchains.py
@@ -1334,7 +1334,7 @@
   parser = commandline.ArgumentParser(description=__doc__)
   parser.add_argument('-u', '--nousepkg',
                       action='store_false', dest='usepkg', default=True,
-                      help='Use prebuilt packages if possible')
+                      help='Do not use prebuilt packages')
   parser.add_argument('-d', '--deleteold',
                       action='store_true', dest='deleteold', default=False,
                       help='Unmerge deprecated packages')