Add `tag` to gruvbox theme (#1555)

Missed in the commit 943fca332e.

runtime/themes/gruvbox.toml

@@ -19,6 +19,7 @@
 "function" = { fg = "green1", modifiers = ["bold"] }
 "function.macro" = "aqua1"
 "function.builtin" = "yellow1"
+"tag" = "red1"
 "comment" = { fg = "gray1", modifiers = ["italic"]  }
 "constant" = { fg = "purple1" }
 "constant.builtin" = { fg = "purple1", modifiers = ["bold"] }