It doesn’t make a difference in the code you posted. In general, the comma separates expressions just like a semicolon, however, if you take the whole as an expression, then the comma operator means that the expression evaluates to the last argument.
Here’s an example:
b = (3, 5);
Will evaluate 3, then 5 and assign the latter to b. So b = 5. Note that the brackets are important here:
b = 3, 5;
Will evaluate b = 3, then 5 and the result of the whole expression is 5, nevertheless b == 3.
The comma operator is especially helpful in for-loops when your iterator code is not a simple i++, but you need to do multiple commands. In that case a semicolon doesn’t work well with the for-loop syntax.