docs: fix custom_areas example

Previously a new highlight was created and cached for each index.
For example when the result table only has a warning, an orange color
highlight will be created and cached for index 1. Then the user fixes
the warning but generates an error with index 1, the orange color will be
reused from the cache rather than creating a highlight for the
associated red color. Using an existing highlight with link will
avoid using that cache entirely.

doc/bufferline.txt

@@ -1126,19 +1126,19 @@ to be shown in a list of tables. For example:
         local hint = #vim.diagnostic.get(0, {severity = seve.HINT})
 
         if error ~= 0 then
-            table.insert(result, {text = "  " .. error, fg = "#EC5241"})
+            table.insert(result, {text = "  " .. error, link = "DiagnosticError"})
         end
 
         if warning ~= 0 then
-            table.insert(result, {text = "  " .. warning, fg = "#EFB839"})
+            table.insert(result, {text = "  " .. warning, link = "DiagnosticWarn"})
         end
 
         if hint ~= 0 then
-            table.insert(result, {text = "  " .. hint, fg = "#A3BA5E"})
+            table.insert(result, {text = "  " .. hint, link = "DiagnosticHint"})
         end
 
         if info ~= 0 then
-            table.insert(result, {text = "  " .. info, fg = "#7EA9A7"})
+            table.insert(result, {text = "  " .. info, link = "DiagnosticInfo"})
         end
         return result
     end,