Slightly modified developers_guide.rst (#19507)

I needed to run not only `./tools/gen_struct_info.py` but also
`./tools/gen_struct_info.py --wasm64`.

#19499 (comment)

Author: mame

site/source/docs/contributing/developers_guide.rst

@@ -158,7 +158,8 @@ If you change the layout of C structs or modify C defines that are used in
 JavaScript library files you may need to modify ``tools/struct_info.json``.  Any
 time that file is modified or a struct layout is changed you will need to run
 ``./tools/gen_struct_info.py`` to re-generate the information used by
-JavaScript.
+JavaScript. Note that you need to run both ``./tools/gen_struct_info.py`` and
+``./tools/gen_struct_info.py --wasm64``.
 
 The ``test_gen_struct_info`` test will fail if you forget to do this.