Update 1137.c

[shivamdroidoreo]

Explanation of Changes: Loop starting from i = 3: This matches the idea that Tribonacci starts at n = 3 for the iterative part. It loops until i = n, making it more aligned with the index you are calculating. Renaming nextT to nextTerm: This improves code readability by being more descriptive.

Notes: <The overall structure of the program is unchanged, but the slight improvements make the code easier to understand and maintain.>