actions: fix documented limit for btrfs labels

Our documentation of set-label says that labels for btrfs are limited
to 256 characters, while btrfs-filesystem(1) says:

  Note
  the maximum allowable length shall be less than 256 chars

which is indeed true, as trying to set a label with 256 characters
fails. Hence, fix our documentation to say that the limit is actually
255 characters.

Related to RHBZ#1164708.

generator/actions.ml

@@ -9979,7 +9979,7 @@ be mounted when trying to set the label.
 
 =item btrfs
 
-The label is limited to 256 bytes and some characters are
+The label is limited to 255 bytes and some characters are
 not allowed.  Setting the label on a btrfs subvolume will set the
 label on its parent filesystem.  The filesystem must not be mounted
 when trying to set the label.