build(gen_helptags): update check for example (neovim#35750)

Follow-up to ae588c1.

Author: zeertzjq

src/gen/gen_helptags.lua

@@ -48,7 +48,7 @@ for _, fn in ipairs(files) do
       end
     end
 
-    if line == '>' or vim.endswith(line, ' >') then
+    if line:find('^>[a-z0-9]*$') or line:find(' >[a-z0-9]*$') then
       in_example = true
     end
   end