Exclude k-wave.org from link checker — site is returning 500

The k-wave.org website is temporarily down (500 Internal Server Error).
Our links are correct; exclude the domain until the site recovers.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: waltsims

.github/workflows/link-check.yml

@@ -30,5 +30,6 @@ jobs:
             --exclude "https://pubs.aip.org"
             --exclude "https://www.gnu.org"
             --exclude "https://fsf.org"
+            --exclude "http://www.k-wave.org"
             "**/*.md" "**/*.rst"
           fail: true