Fix missing comma in GRPO equation

[ab490]

This PR fixes a missing comma in the GRPO objective equation in Chapter 12.

The comma is added after the first (A_i), clearly separating the two arguments of the min(...) function.

The corrected equation is:

$$J_{GRPO}(\theta) = \left[\frac{1}{G} \sum_{i=1}^{G} \min \left( \frac{\pi_{\theta}(o_i|q)}{\pi_{\theta_{old}}(o_i|q)} A_i, \text{clip}\left( \frac{\pi_{\theta}(o_i|q)}{\pi_{\theta_{old}}(o_i|q)}, 1 - \epsilon, 1 + \epsilon \right) A_i \right)\right]- \beta D_{KL}(\pi_{\theta} || \pi_{ref})$$

Fixes #1174