fix small_fib example for indices of 2^x

[ph4r05]

This PR fixes small_fib example for indices of format 2^x.

When testing claims with index: 2^x, x >=1, value 1 passes the proof verification. I think the reason is how the trace is constructed.

Trace length is defined as power of two:

let trace_length = self.index.next_power_of_two();

But if self.index is already power of two, trace_length == self.index. It can be a bit misleading that next keyword returns greater or equal power of two

Thus if I change it to

let trace_length = (self.index + 1).next_power_of_two();

all indices work fine now.

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