Clarify that plugin IDs must be lowercase (#820)

api/src/ap/java/com/velocitypowered/api/plugin/ap/PluginAnnotationProcessor.java

@@ -76,8 +76,8 @@ public class PluginAnnotationProcessor extends AbstractProcessor {
       if (!SerializedPluginDescription.ID_PATTERN.matcher(plugin.id()).matches()) {
         environment.getMessager().printMessage(Diagnostic.Kind.ERROR, "Invalid ID for plugin "
             + qualifiedName
-            + ". IDs must start alphabetically, have alphanumeric characters, and can "
-            + "contain dashes or underscores.");
+            + ". IDs must start alphabetically, have lowercase alphanumeric characters, and "
+            + "can contain dashes or underscores.");
         return false;
       }