Fix typo in vignette: $epix_merge -> $merge

Author: lcbrooks

vignettes/archive.Rmd

@@ -263,7 +263,7 @@ y <- covidcast(
   select(geo_value, time_value, version = issue, case_rate_7d_av = value) %>%
   as_epi_archive(compactify=TRUE)
 
-x$epix_merge(y)
+x$merge(y)
 print(x)
 head(x$DT)
 ```