remove unnecessary [ ] from datatable-keys-fast-subset.Rmd

vignettes/datatable-keys-fast-subset.Rmd

@@ -108,7 +108,7 @@ Instead, in *data.tables* we set and use `keys`. Think of a `key` as **superchar
 
 #### Keys and their properties {#key-properties}
 
-1. [ ] We can set keys on *multiple columns* and the column can be of *different types* -- *integer*, *numeric*, *character*, *factor*, *integer64* etc. *list* and *complex* types are not supported yet.
+1. We can set keys on *multiple columns* and the column can be of *different types* -- *integer*, *numeric*, *character*, *factor*, *integer64* etc. *list* and *complex* types are not supported yet.
 
 2. Uniqueness is not enforced, i.e., duplicate key values are allowed. Since rows are sorted by key, any duplicates in the key columns will appear consecutively.