WASM clarified --with-wasm-module

Option is for WASM only, so shouldn't be named --with-main-module.

Also the option's default value changed since it was introduced, so the
documentation became outdated. See commits:

fe678ffc6f Emscripten: No need to pass --with-main-module into build configure
437b4d802c Fix default value of Emscripten --with-main-module
8aef8c3a14 Make it possible to run either Writer or Calc in the same Emscripten thing
97ee126d69 WASM add Calc as optional build result, make it build & run

Change-Id: I29aeeb9a840ec07e46cbf153c535b3fca9d88cd1
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/182798
Tested-by: Jenkins
Reviewed-by: Moritz Duge <moritz.duge@allotropia.de>

static/README.wasm.md

@@ -433,7 +433,7 @@ For instance, this autogen.input works for me:
 `--disable-crashdump`
 `--host=wasm32-local-emscripten`
 `--disable-gui`
-`--with-main-module=writer`
+`--with-wasm-module=writer`
 `--with-package-format=emscripten`
 
 For building LO core for use in COWASM, it is known to work to use