Fix typo in regression.qmd regarding varying effects

Author: emstruong

src/stan-users-guide/regression.qmd

@@ -529,7 +529,7 @@ effect $\beta_k$ for age group $k$ to make a linear predictor $\alpha +
 subtract from each $\beta_k$ and get exactly the same likelihood.
 
 The traditional approach to identifying such a model is to pin the
-first varing effect to zero, i.e., $\beta_1 = 0$.  With one of the
+first varying effect to zero, i.e., $\beta_1 = 0$.  With one of the
 varying effects fixed, you can no longer add a constant to all of them
 and the model's likelihood is identified.  In addition to the
 difficulty in specifying such a model in Stan, it is awkward to