Fix prerequisites check in webui.sh

- Check the actually used `$python_cmd` and `$GIT` executables instead
  of the hardcoded ones
- Fix typo in comment

webui.sh

@@ -82,8 +82,8 @@ then
     clone_dir="${PWD##*/}"
 fi
 
-# Check prequisites
+# Check prerequisites
-for preq in git python3
+for preq in "${GIT}" "${python_cmd}"
 do
     if ! hash "${preq}" &>/dev/null
     then