Update datatable-fread-and-fwrite.Rmd #7253 VignetteIndexEntry

change of VignetteIndexEntry following aitap recommendation

Author: ChristianWia

vignettes/fr/datatable-fread-and-fwrite.Rmd

@@ -4,7 +4,7 @@ date: "`r Sys.Date()`"
 output: 
   markdown::html_format
 vignette: >
-  %\VignetteIndexEntry{Importing data.table}
+  %\VignetteIndexEntry{Fast Read and Fast Write}
   %\VignetteEngine{knitr::knitr}
   \usepackage[utf8]{inputenc}
 ---