Add "-DPLATFORM=Web" with cmake example

Working-for-Web-(HTML5).md

@@ -114,7 +114,7 @@ If you prefer to use `CMake` instead of the plain `Makefile` provided, you have
 **You can go with the [emscripten suggested way](https://emscripten.org/docs/compiling/Building-Projects.html)**: That is to use the following command that will add the compiler and toolchain options for you:
 
 ```
-emcmake cmake -H . -B build
+emcmake cmake -H . -B build 
 ```
 _(The "-H ." is deprecated since CMake 3.13 and if you are using higher version please replace the argument with "-S .")_
 
@@ -123,7 +123,7 @@ Emscripten cmake will prefer the [ninja build system](https://ninja-build.org/)
 **IDE-friendly option**: As the first option is only available from a command line where you have run "emsdk activate latest" when using an IDE (like Visual Studio, CLion, etc.) you should set the path to the emscripten toolchain file:
 
 ```
-cmake -H . -B build -G Ninja "-DCMAKE_TOOLCHAIN_FILE=<fullpath_to_emsdk>/upstream/emscripten/cmake/Modules/Platform/Emscripten.cmake"
+cmake -H . -B build -G Ninja -DPLATFORM=Web "-DCMAKE_TOOLCHAIN_FILE=<fullpath_to_emsdk>/upstream/emscripten/cmake/Modules/Platform/Emscripten.cmake"
 ```
 _(The "-H ." is deprecated since CMake 3.13 and if you are using higher version please replace the argument with "-S .")_
 _(The ninja generator is optional and you can use your system default by removing "-G Ninja".)_