Fix mis-documentation

doc/blockdev.md

@@ -20,7 +20,7 @@ That tells `blockdev` that we're going to set up one device, that its GetC and
 PutC are the ones defined by `acia.asm`.
 
 If your block device is read-only or write-only, use dummy routines. `unsetZ`
-is a good choice since it will return with the `Z` flag set, indicating an
+is a good choice since it will return with the `Z` flag unset, indicating an
 error (dummy methods aren't supposed to be called).
 
 Each defined block device, in addition to its routine definition, holds a
@@ -33,12 +33,12 @@ they should try to adhere to the convention, that is:
 
 **GetC**: Get the character at position specified by `HL`. If it supports 32-bit
           addressing, `DE` contains the high-order bytes. Return the result in
-          `A`. If there's an error (for example, address out of range), set `Z`.
-          This routine is not expected to block. We expect the result to be
+          `A`. If there's an error (for example, address out of range), unset
+          `Z`. This routine is not expected to block. We expect the result to be
           immediate.
 
 **PutC**: The opposite of GetC. Write the character in `A` at specified
-          position. `Z` set on error.
+          position. `Z` unset on error.
 
 ## Shell usage