fix some warnings (#660)

Co-authored-by: ArthurSonzogni <sonzogniarthur@gmail.com>

examples/dom/canvas.cpp

@@ -8,7 +8,7 @@
 #include "ftxui/dom/node.hpp"    // for Render
 #include "ftxui/screen/color.hpp"  // for Color, Color::Red, Color::Blue, Color::Green, ftxui
 
-int main(int argc, const char* argv[]) {
+int main() {
   using namespace ftxui;
 
   auto c = Canvas(100, 100);