Fix prime symbol

The correct is

~~~
<msup><mi>f</mi><mo>'</mo></msup>
~~~

instead of

~~~
<mi>f</mi><mo>'</mo>
~~~

More information at https://bugzilla.mozilla.org/show_bug.cgi?id=442637.
Merge pull request #34 from r-gaia-cs/fix-opp

Fix prime symbol