Add second entry for GL_ENABLE_GET_PROC_ADDRESS=1

Working-for-Web-(HTML5).md

@@ -147,7 +147,7 @@ cmake --build build
 
 ```cmake
 if (EMSCRIPTEN)
-    set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -s USE_GLFW=3 -s ASSERTIONS=1 -s WASM=1 -s ASYNCIFY")
+    set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -s USE_GLFW=3 -s ASSERTIONS=1 -s WASM=1 -s ASYNCIFY -s GL_ENABLE_GET_PROC_ADDRESS=1")
     set(CMAKE_EXECUTABLE_SUFFIX ".html") # This line is used to set your executable to build with the emscripten html template so that you can directly open it.
 endif ()
 ```