fix typo in function help

Co-authored-by: Miod Vallat <[email protected]>

lib/overlayfs_store.mli

@@ -3,5 +3,5 @@
 include S.STORE
 
 val create : path:string -> t Lwt.t
-(** [create ~path] creates a new xfs store where everything will
+(** [create ~path] creates a new overlayfs store where everything will
     be stored under [path]. *)