Optimized the cost function and adjusted alpha for Multi variable example

[arpitparmar5739]

1. Optimized the code in the cost function of the multi variable cost function file for better performance.
2. Changed the value of alpha in the Multi variable ex1_multi.m file for better results.

Also earlier number of iterations required were greater than 400 but the value of num_iters was 400.
Now, the number of iterations can also be changed to a lesser value (smaller than 400).