Update R/psis.R

Co-authored-by: Jonah Gabry <jgabry@gmail.com>

Author: VisruthSK

R/psis.R

@@ -213,7 +213,7 @@ do_psis_i <- function(log_ratios_i, tail_len_i, ...) {
   # shift log ratios for safer exponentation
   lw_i <- log_ratios_i - max(log_ratios_i)
 
-  if (length(unique(tail(log_ratios_i, -tail_len_i))) == 1) {
+  if (length(unique(utils::tail(log_ratios_i, -tail_len_i))) == 1) {
     warning(
       "Can't fit generalized Pareto distribution ",
       "because all tail values are the same.",