Fix VtRread typo and remove reference to vbackup(8) in venti(6).

Reference: /n/sources/patch/applied/man-6-venti-vtrread
Date: Sun Mar 18 18:50:47 CET 2012
Signed-off-by: djc@9grid.fr

--- /sys/man/6/venti	Sun Mar 18 18:50:23 2012
+++ /sys/man/6/venti	Sun Mar 18 18:50:15 2012
@@ -42,13 +42,7 @@
 .IR vac (1)
 uses a
 .B vac:
-prefix, while
-.IR vbackup (8)
-uses prefixes corresponding to the file system
-types: 
-.BR ext2: ,
-.BR ffs: ,
-and so on.
+prefix.
 .SS "Files and Directories
 Venti accepts blocks up to 56 kilobytes in size.  
 By convention, Venti clients use hash trees of blocks to
@@ -292,7 +286,7 @@
 .IR pad [1]
 .IR count [2]
 .br
-.B VtRead
+.B VtRread
 .IR tag [1]
 .IR data []
 .IP