tweak fs.h spacing; add comment

fs.h

@@ -46,6 +46,8 @@ struct dinode {
 // Block containing bit for block b
 #define BBLOCK(b, ninodes) (b/BPB + (ninodes)/IPB + 3)
 
+// PAGEBREAK: 10
+// Directory is a file containing a sequence of dirent structures.
 #define DIRSIZ 14
 
 struct dirent {