syntax: fix broken error message in previous patch

syntax: fix broken error message in previous patch

Osier Yang pointed out that I introduced a syntax error in my
syntax check (I really shouldn't make last-minute changes without
testing them....).

/bin/sh: -c: line 2: syntax error near unexpected token `;'
/bin/sh: -c: line 2: `  { echo 'maint.mk: incorrect whitespace, see HACKING for rules' 2>&; \'
make: *** [bracket-spacing-check] Error 1

* cfg.mk (bracket-spacing-check): Fix copy-and-paste error.

Signed-off-by: Eric Blake <eblake@redhat.com>