doc: fix missing argument for setting python path

lagorsse · rvagg · commit 785e527c3df4 · 2019-09-26T13:39:38.000+10:00
(accepted with minor modifications by rvagg) PR-URL: #1802 Reviewed-By: Rod Vagg <rod@vagg.org>
diff --git a/README.md b/README.md
@@ -63,7 +63,7 @@ If you have multiple Python versions installed, you can identify which Python
 version `node-gyp` uses by setting the `--python` variable:
 
 ``` bash
-$ node-gyp --python /path/to/python2.7
+$ node-gyp <command> --python /path/to/executable/python2.7
 ```
 
 If `node-gyp` is called by way of `npm`, *and* you have multiple versions of
