Improve HMC parameters for faster convergence (#570)

Co-authored-by: Penelope Yong <penelopeysm@gmail.com>

tutorials/docs-12-using-turing-guide/index.qmd

@@ -280,7 +280,7 @@ end
 
 # Construct a model with x = missing
 model = gdemo(missing)
-c = sample(model, HMC(0.01, 5), 500)
+c = sample(model, HMC(0.05, 20), 500)
 ```
 
 Note the need to initialize `x` when missing since we are iterating over its elements later in the model. The generated values for `x` can be extracted from the `Chains` object using `c[:x]`.