Fix incorrect statement in GammaPoisson log_prob docstring (#2194)

The mathematical expression is for the unlogged pmf, not the log pmf.

Author: dylanhmorris

numpyro/distributions/conjugate.py

@@ -383,7 +383,7 @@ def sample(self, key: jax.Array, sample_shape: tuple[int, ...] = ()) -> ArrayLik
 
     @validate_sample
     def log_prob(self, value: ArrayLike) -> ArrayLike:
-        r"""If :math:`X \sim \mathrm{GammaPoisson}(\alpha, \lambda)`, then the log
+        r"""If :math:`X \sim \mathrm{GammaPoisson}(\alpha, \lambda)`, then the
         probability mass function is:
 
         .. math::