fix typo in lbeta(): replace a and b with \alpha and \beta

Author: mikegilchrist

src/functions-reference/real-valued_basic_functions.Rmd

@@ -1217,7 +1217,7 @@ Return the natural logarithm of the beta function applied to alpha and
 beta. The beta function, $\text{B}(\alpha,\beta)$, computes the
 normalizing constant for the beta distribution, and is defined for
 $\alpha > 0$ and $\beta > 0$. \[ \text{lbeta}(\alpha,\beta) = \log
-\Gamma(a) + \log \Gamma(b) - \log \Gamma(a+b) \] See section
+\Gamma(\alpha) + \log \Gamma(\beta) - \log \Gamma(\alpha+\beta) \] See section
 [appendix](#beta-appendix) for definition of $\text{B}(\alpha, \beta)$.
 `r since("2.0")`