Fix a typo roboto, code had ".tff", instead of the correct ".ttf"

[grepsuzette]

Here are the steps to identify the problem:

1) In Firefox debug console with some website, this problem creates warnings such as downloadable font: no supported format found (font-family: "Roboto" style:normal weight:400 stretch:100 src index:1) source: (end of source list)

2) In Stylesheet editor this manifests itself as something like:

    @font-face {
      font-family: Roboto; -moz-osx-font-smoothing: grayscale; -webkit-font-smoothing: antialiased;
      font-style: normal; font-weight: normal; src: url(moz-extension://93af..............93c24/Roboto-Regular.tff) format('tff');
    }

3) Opening moz-extension://93af..............93c24/Roboto-Regular.tff gives an error 4) Opening moz-extension://93af..............93c24/Roboto-Regular.ttf gives a font!

Therefore I suggest this fix. Hopefully it will be applied even if it's not an urgent PR.

[eejdoowad]

Thanks for the fix!