doubt in pipelining forwarding in mips
The reason for using 63 instead of 64 in the calculation is simple.
Instruction that was fetched after bnez has to be flushed everytime, except for the last iteration.
For the last iteration, this instruction does not have to be flushed and saves one cycle of delay (as it is mentioned in the linked pdf), so we have the delay calculation as (6+2) instead of (6+3) for the last iteration.

Calculations:
There are 6 instructions in the loop. The Cycles Per Instruction (CPI) of the pipelined processor is 1. There are 2 stalls and 1 cycle is required to flush the instruction after bnez.
Total cycle count = (Number of instructions x CPI) + Number of stalls + Number of cycles required to flush the instruction after bnez
= (6x1) + 2 + 1
For last iteration the flush is not required. So it becomes 6+2.

Some free online courses to learn more about this topic:

https://learn.saylor.org/course/view.php?id=71
https://www.coursera.org/learn/comparch/