Simplify FrameRateType RegExp

Minor modification for the `FrameRateType` descriptor.

Based on my general knowledge and [tutorials](http://www.regular-expressions.info/repeat.html) for regular expression it is safe to say these two patterns are equivalent to each other.
But I couldn't find limitation in MPD, neither could use the binaries in the repository to verify this assumption. It would be great to check this before the merge.