pw_env_setup: Fix help string for argument

Change-Id: I060b28dc735caabf018443d060d9f4bc45bbe290
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/24180
Pigweed-Auto-Submit: Rob Mohr <mohrr@google.com>
Reviewed-by: Alexei Frolov <frolv@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/pw_env_setup/py/pw_env_setup/env_setup.py b/pw_env_setup/py/pw_env_setup/env_setup.py
index c5c23fb..653af47 100755
--- a/pw_env_setup/py/pw_env_setup/env_setup.py
+++ b/pw_env_setup/py/pw_env_setup/env_setup.py
@@ -552,7 +552,7 @@
 
     parser.add_argument(
         '--virtualenv-root',
-        help=('Basename of virtualenv directory. Default: '
+        help=('Root of virtualenv directory. Default: '
               '<install_dir>/pigweed-venv'),
         default=None,
     )