Fix matching parens background color in GUI mode

The background color for matching parens (group "MatchParen") has been
improved in version "0.7.0" (1), but instead of using "nord3" as
background color in GUI mode "nord0" has been assigned.

References:

  (1) https://github.com/arcticicestudio/nord-vim/projects/10

GH-95

colors/nord.vim

@@ -139,7 +139,7 @@ call s:hi("CursorLine", "", s:nord1_gui, "NONE", s:nord1_term, "NONE", "")
 call s:hi("Error", s:nord0_gui, s:nord11_gui, "", s:nord11_term, "", "")
 call s:hi("iCursor", s:nord0_gui, s:nord4_gui, "", "NONE", "", "")
 call s:hi("LineNr", s:nord3_gui, s:nord0_gui, s:nord3_term, "NONE", "", "")
-call s:hi("MatchParen", s:nord8_gui, s:nord0_gui, s:nord8_term, s:nord3_term, "", "")
+call s:hi("MatchParen", s:nord8_gui, s:nord3_gui, s:nord8_term, s:nord3_term, "", "")
 call s:hi("NonText", s:nord2_gui, "", s:nord3_term, "", "", "")
 call s:hi("Normal", s:nord4_gui, s:nord0_gui, "NONE", "NONE", "", "")
 call s:hi("PMenu", s:nord4_gui, s:nord2_gui, "NONE", s:nord1_term, "NONE", "")