README: use standard git mechanism for submodules

As part of the history rewrite we removed all unused micropython
submodules, so we don't have to use their machinery anymore to only
update submodules that are in use by our port.

README.md

@@ -51,7 +51,7 @@ Standard ESP-IDF project machinery present and working. You can run `idf.py` fro
 Prepare submodules:
 
 ```
-$ make -C micropython/ports/esp32 submodules
+git submodule update --init --recursive
 ```
 
 Build normally with idf.py: