Update usage strings to put flags before args

The standard UNIX-style ordering of command-line arguments places
optional flags before other positional arguments. All of restic's
commands support this ordering, but some of the usage strings showed the
flags after the positional arguments (which restic also parses just
fine). This change updates the doc strings to reflect the standard
ordering.

Because the `restic help` command comes directly from Cobra, there does
not appear to be a way to update the argument ordering in its usage
string, so it maintains the non-standard ordering (positional arguments
before optional flags).

doc/020_installation.rst

@@ -292,7 +292,7 @@ Restic can write out man pages and bash/zsh compatible autocompletion scripts:
     and the auto-completion files for bash and zsh).
 
     Usage:
-      restic generate [command] [flags]
+      restic generate [flags] [command]
 
     Flags:
           --bash-completion file   write bash completion file