Backends: OpenGL3: Commented out extra tokens at end of #else directive (#2804)

examples/imgui_impl_opengl3.cpp

@@ -172,7 +172,7 @@ bool    ImGui_ImplOpenGL3_Init(const char* glsl_version)
     gl_loader = "GLEW";
 #elif defined(IMGUI_IMPL_OPENGL_LOADER_GLAD)
     gl_loader = "GLAD";
-#else IMGUI_IMPL_OPENGL_LOADER_CUSTOM
+#else // IMGUI_IMPL_OPENGL_LOADER_CUSTOM
     gl_loader = "Custom";
 #endif