[improve log info] fix typo (#2520)

diff --git a/bookkeeper-server/src/main/java/org/apache/bookkeeper/bookie/Bookie.java b/bookkeeper-server/src/main/java/org/apache/bookkeeper/bookie/Bookie.java
index 7f079f4..a8430c9 100644
--- a/bookkeeper-server/src/main/java/org/apache/bookkeeper/bookie/Bookie.java
+++ b/bookkeeper-server/src/main/java/org/apache/bookkeeper/bookie/Bookie.java
@@ -1573,7 +1573,7 @@
         if (null != indexDirs) {
             for (File dir : indexDirs) {
                 if (!cleanDir(dir)) {
-                    LOG.error("Formatting ledger directory " + dir + " failed");
+                    LOG.error("Formatting index directory " + dir + " failed");
                     return false;
                 }
             }