Fix flutter root error message string interpolation (flutter#24944)

The variable isn't interpolating because it's using "${}" when ruby uses "#{}".

Author: seanfreiburg

packages/flutter_tools/templates/module/ios/library/Flutter.tmpl/podhelper.rb

@@ -23,7 +23,7 @@ def parse_KV_file(file, separator='=')
 def flutter_root(f)
     generated_xcode_build_settings = parse_KV_file(File.join(f, File.join('.ios', 'Flutter', 'Generated.xcconfig')))
     if generated_xcode_build_settings.empty?
-        puts "Generated.xcconfig must exist. Make sure `flutter packages get` is executed in ${f}."
+        puts "Generated.xcconfig must exist. Make sure `flutter packages get` is executed in #{f}."
         exit
     end
     generated_xcode_build_settings.map { |p|