Use String.replaceFirst() to substitute %s with username

src/main/java/org/typo3/gerrit/plugins/avatars/typo3org/ExternalUrlAvatarProvider.java

diff --git a/src/main/java/org/typo3/gerrit/plugins/avatars/typo3org/ExternalUrlAvatarProvider.java b/src/main/java/org/typo3/gerrit/plugins/avatars/typo3org/ExternalUrlAvatarProvider.java
index 5569873..6545b2b 100644
--- a/src/main/java/org/typo3/gerrit/plugins/avatars/typo3org/ExternalUrlAvatarProvider.java
+++ b/src/main/java/org/typo3/gerrit/plugins/avatars/typo3org/ExternalUrlAvatarProvider.java
@@ -75,7 +75,7 @@
     }
 
     // replace the %s inside the URL with the username
-    String url = String.format(externalAvatarUrl, username);
+    String url = externalAvatarUrl.replaceFirst("%s", username);
 
     return url;
   }