Update README.md

fix link

Author: lurch

linux/filesystem/README.md

@@ -5,5 +5,5 @@ It is important to have a basic understanding of the fundamentals of the Linux f
 # Contents
 
 - [Home](home.md)
-- [Backups](backups.md)
+- [Backups](backup.md)
 - [Whole filesystem](whole-filesystem.md)