Follow PEP 394 in build_mozc_for_android.md

This follows up to my previous commit [1], which introduced

  docs/build_mozc_for_android.md

as a dedicated build instruction page for building 'libmozc.so' for
Android as part of removing the dependency on Docker from our build
instructions (google#1181).

While the above document uses 'python3' elsewhere, there was one place
where 'python' was used instead. It would be better to avoid 'python'
in favor of PEP 394 [2].

 [1]: f215eaf
 [2]: https://peps.python.org/pep-0394/

docs/build_mozc_for_android.md

@@ -69,7 +69,7 @@ Using `mozc/src/python-venv` as the virtual environment location is not mandator
 ### Check out additional build dependencies
 
 ```
-python build_tools/update_deps.py
+python3 build_tools/update_deps.py
 ```
 
 In this step, additional build dependencies will be downloaded.