Fix asymmetric_irt.qmd render crash: avoid expression() labs with ggplotly

expression(theta)/expression(P(theta)) axis labels broke plotly's
verify_attr (unique.default() doesn't support type 'expression'),
halting render at fig-example-irf. Swapped for plain Unicode strings.

Author: ben-domingue

vignettes/asymmetric_irt.qmd

@@ -811,7 +811,7 @@ p <- ggplot(ex_df, aes(x = theta, y = p, colour = curve, group = curve, text = t
   ) +
   scale_colour_manual(values = c("LPE" = irw_blue, "Symmetric (kappa=1)" = irw_grey), name = NULL) +
   scale_size_continuous(range = c(1.5, 5), guide = "none") +
-  labs(x = expression(theta), y = expression(P(theta)))
+  labs(x = "θ", y = "P(θ)")
 
 ggplotly(p, tooltip = "text") |>
   layout(legend = list(orientation = "h", y = -0.15))