Fix Javadoc error on Java 13.
diff --git a/src/main/java/org/apache/commons/text/AlphabetConverter.java b/src/main/java/org/apache/commons/text/AlphabetConverter.java
index 8cc1af6..9449904 100644
--- a/src/main/java/org/apache/commons/text/AlphabetConverter.java
+++ b/src/main/java/org/apache/commons/text/AlphabetConverter.java
@@ -45,7 +45,7 @@
  * chars, which will be of length 1
  * </p>
  *
- * <h1>Sample usage</h1>
+ * <h2>Sample usage</h2>
  *
  * <pre>
  * Character[] originals;   // a, b, c, d