DashboardBuilders: clarify the Python 3 binary name requirement

The bootstrapswarm command, among others, expect that Python 3 is
available under the exact name "python3". In some cases Python 3
might be available as "python" or "python3.11" and so on, but it's
likely better to standardize on a single name. Clarify what it is.

Change-Id: I651c27ec61ddfc2398447ee93706acf0dfbf88b8
Reviewed-on: https://go-review.googlesource.com/c/wiki/+/653735
Reviewed-by: Carlos Amedee <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>

Author: dmitshur

DashboardBuilders.md

@@ -21,7 +21,7 @@ The LUCI system requires builders to run two applications which authenticate to
     - At least 512MB of memory. 1GB or more is highly recommended.
     - 20GB disk space is ideal.
     - Preferably with 2 or more (V)CPUs.
-  * Python3 installed and in the `PATH`.
+  * Python 3 installed and available in `PATH` (or equivalent) under the name `python3` (or `python3.exe` on Windows).
   * Permissions
     - The bot should be run as the `swarming` user (without root rights).
     - The bot automatically updates itself. It should have permissions to do so.