Use the py command from Python Launcher for Windows in tutorial (#9338)

Co-authored-by: Thibaud Colas <thibaudcolas@gmail.com>

docs/getting_started/tutorial.md

@@ -14,7 +14,13 @@ Wagtail supports Python 3.7, 3.8, 3.9 and 3.10.
 To check whether you have an appropriate version of Python 3:
 
 ```sh
-python3 --version
+python --version
+```
+
+**On Windows** (cmd.exe, with the Python Launcher for Windows):
+
+```sh
+py --version
 ```
 
 If this does not return a version number or returns a version lower than 3.7, you will need to [install Python 3](https://www.python.org/downloads/).
@@ -33,7 +39,7 @@ This tutorial uses [`venv`](https://docs.python.org/3/tutorial/venv.html), which
 **On Windows** (cmd.exe):
 
 ```doscon
-python3 -m venv mysite\env
+py -m venv mysite\env
 mysite\env\Scripts\activate.bat
 # or:
 mysite\env\Scripts\activate
@@ -42,7 +48,7 @@ mysite\env\Scripts\activate
 **On GNU/Linux or MacOS** (bash):
 
 ```sh
-python3 -m venv mysite/env
+python -m venv mysite/env
 source mysite/env/bin/activate
 ```