use slice_head instead of head for potential tibbles

Change-Id: I1ce68ed52209ebaf22e0e29df9ac7281d83e4bc8
diff --git a/R/collocationAnalysis.R b/R/collocationAnalysis.R
index a2d6c5d..d933e83 100644
--- a/R/collocationAnalysis.R
+++ b/R/collocationAnalysis.R
@@ -144,7 +144,7 @@
               if (nrow(candidates) > 0) {
                 candidates <- candidates %>%
                   filter(frequency >= minOccur) %>%
-                  head(topCollocatesLimit)
+                  slice_head(n=topCollocatesLimit)
                 collocationScoreQuery(
                   kco,
                   node = node,