doc: build rules requires /usr/bin/python

maven_jar would fail to fetch material when a `python` executable is not
found in path. It looks it up with:

 python = ctx.which("python")

Since Debian 11 and Ubuntu 20.04 LTS, there is no more a
`/usr/bin/python` to get rid of the ambiguity between python 2 and
python 3.

8cc50228 erroneously mentionned the build rule requires python 2,
instead of the lack of `python`.  Adjust the document to instruct people
to install Python 3.

Change-Id: I455edc28f1b67663409ee1c612edd21a5cc57720
diff --git a/Documentation/ b/Documentation/
index 82fedb0..70b050e 100644
--- a/Documentation/
+++ b/Documentation/
@@ -36,11 +36,12 @@
 Error in execute: Argument 0 of execute is neither a path, label, nor string.
-Make sure you are using python2. For debian users, the command
+Make sure you have `python` available in your PATH. Since Debian 11 and Ubuntu
+20.04 LTS (focal), there is no `/usr/bin/python` provided by default. You can
+install a package to provide a link to Python 3:
-sudo apt-get install python-is-python2
+sudo apt-get install python-is-python3
 Upon uploading your new CL, if you encounter a message related to a