convert spoiler code block to executable chunk in contact-matrices

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: avallecam

episodes/contact-matrices.Rmd

@@ -148,7 +148,7 @@ zambia_sa_survey <- socialmixr::load_survey(files = zambia_survey_files)
 
 You can explore all the available surveys from the Zenodo repository at <https://zenodo.org/communities/social_contact_data/>. If you are interested in accessing to a specific URL within R, you can try:
 
-```r
+```{r}
 library(contactsurveys)
 library(tidyverse)