Use two chains as a work-around for stan-dev/cmdstanr#1050.

getting_started/getting_started.Rmd

@@ -21,7 +21,7 @@ model <- cmdstan_model(
 )
 fit <- model$sample(
   data = list(n = 100, sigma = 1, length_scale = 0.1, period = 1),
-  chains = 1,
+  chains = 2,
   iter_warmup = 500,
   iter_sampling = 50
 )