minor tweak to fix a .dotfile that yeoman got grumpy about

Author: yochannah

generators/app/index.js

@@ -103,7 +103,7 @@ module.exports = class extends Generator {
       this.destinationPath("dev/serve.js"));
 
     this.fs.copyTpl(
-      this.templatePath(".gitignore"),
+      this.templatePath("_gitignore"),
       this.destinationPath(".gitignore"));
 
     this.fs.copyTpl(