Add cppreference link to ignore list (UniversalRobots#506)

The workflow gets a 403 on this

Author: urfeex

.github/workflows/check_links.yml

@@ -36,6 +36,7 @@ jobs:
             --exclude '2013181\/gdb-causes-sem-wait-to-fail-with-eintr-error'
             --exclude 'https:\/\/lists\.apple\.com\/archives\/darwin-kernel\/2009\/Apr\/msg00010\.html'
             --exclude 'https:\/\/wiki\.linuxfoundation\.org\/realtime\/preempt_rt_versions'
+            --exclude 'https:\/\/en\.cppreference\.com\/w\/cpp\/string\/basic_string\/to_string'
             --max-concurrency 1
             './**/*.md' './**/*.html' './**/*.rst' './**/*.cpp' './**/*.h' './**/*.py'
       - name: Save lychee cache