Fix ar1_turningpts build: drop figsize from az.plot_trace (#930)

arviz 1.x dropped the figsize kwarg on plot_trace, which raises
ValueError and fails notebook execution once the build cache is
invalidated. Remove figsize from the az.plot_trace call (arviz uses its
default size). Verified against arviz 1.2.0 that plot_trace(idata) and
trace.posterior.rho access both still work.

Closes #929

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: jstac

lectures/ar1_turningpts.md

@@ -328,7 +328,7 @@ def draw_from_posterior(sample):
 
     # check condition
     with AR1_model:
-        az.plot_trace(trace, figsize=(17, 6))
+        az.plot_trace(trace)
     
     rhos = trace.posterior.rho.values.flatten()
     sigmas = trace.posterior.sigma.values.flatten()