Fix Windows curl command percent-sign escaping

The Windows curl command was using `%%%%{http_code}` which after
Lua's string.format() became `%%{http_code}`. This worked in cmd.exe
batch files but failed in Git Bash and direct io.popen() execution,
where the double-percent was interpreted literally.

Changed to `%%{http_code}` (same as Unix) since io.popen() executes
commands directly, not through a batch file interpreter.

Closes #1

Author: coatless

_extensions/linkrot/linkrot.lua

@@ -145,7 +145,7 @@ end
 local function build_windows_curl_command(url)
   local escaped_url = url:gsub('"', '""')
   return string.format(
-    'curl.exe -s -o NUL -w "%%%%{http_code}" -L --max-time %d -A "%s" "%s" 2>NUL',
+    'curl.exe -s -o NUL -w "%%{http_code}" -L --max-time %d -A "%s" "%s" 2>NUL',
     config.timeout,
     config.user_agent,
     escaped_url