Update rapid_prototyping.rst

Fixed a few grammar mistakes
A construct beginning with "It is where ..." requires a continuation of "that ...". For example, "It is where the traffic lights are that I can cross the road.". Without the continuation, the subject 'It' must be defined in the preceding sentence. In this case it is ambiguous since the preceding sentence also contains two object. In this scenario the subject must be explicit, hence the repeating of "This".

Years of teaching English ;)