Fix PatternLayout.Builder#withHeader() javadoc (#1411)

Author: mernst

log4j-core/src/main/java/org/apache/logging/log4j/core/layout/PatternLayout.java

@@ -744,7 +744,7 @@ public Builder withNoConsoleNoAnsi(final boolean noConsoleNoAnsi) {
 
         /**
          * @param header
-         *        The footer to place at the top of the document, once.
+         *        The header to place at the top of the document, once.
          */
         public Builder withHeader(final String header) {
             this.header = header;