btrfs-progs: docs: fix 'group' typo in btrfsrune page

Pull-request: #634
Signed-off-by: Jonathan Liu <net147@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: net147

Documentation/btrfstune.rst

@@ -28,7 +28,7 @@ OPTIONS
         (since kernel 6.1)
 
         Convert portions of extent tree that tracks block groups to a separate
-        block gruop tree. This greatly reduces mount time. Can be also enabled
+        block group tree. This greatly reduces mount time. Can be also enabled
         at mkfs time.
 
 --convert-from-block-group-tree