Update R.gitignore (#3034)

ignore knitr and R markdown default cache directories in subdirectories

R.gitignore

@@ -28,7 +28,7 @@ vignettes/*.pdf
 .httr-oauth
 
 # knitr and R markdown default cache directories
-/*_cache/
+*_cache/
 /cache/
 
 # Temporary files created by R markdown