Address luacheck warning

"lua/scrollview/signs/keywords.lua:128:25: value assigned to variable
match is unused"

Author: dstein64

lua/scrollview/signs/keywords.lua

@@ -143,8 +143,10 @@ function M.init(enable)
                         -- Treesitter nor 'commentstring'.
                         if scope == SCOPE_AUTO then
                           match = in_comment_ts == nil and in_comment_cs == nil
-                        else  -- scope == SCOPE_COMMENTS
+                        elseif scope == SCOPE_COMMENTS then
                           match = false
+                        else
+                          error('Unknown scope: ' .. scope)
                         end
                       end
                     end