brackets: Highlight closing ')' of array assignments correctly. (Issue #226.)

Followup to c0dafd1d85, which changed the highlight of
those ')'s from <nothing> to [reserved-word].

highlighters/main/test-data/assign.zsh

@@ -32,5 +32,7 @@ BUFFER='A=1 b=("foo" bar)'
 
 expected_region_highlight=(
   "1 3 $ZSH_HIGHLIGHT_STYLES[assign]" # A=1
+  "5 7 $ZSH_HIGHLIGHT_STYLES[assign]" # b=(
   "8 12 $ZSH_HIGHLIGHT_STYLES[double-quoted-argument]" # "foo"
+  "17 17 $ZSH_HIGHLIGHT_STYLES[assign]" # )
 )