Fix typo

src/main/java/net/openhft/chronicle/map/ChronicleMapBuilder.java

@@ -1013,7 +1013,7 @@ public final class ChronicleMapBuilder<K, V> implements
     long entries() {
         if (entries < 0) {
             throw new IllegalStateException("If in-memory Chronicle Map is created or persisted\n" +
-                    "to a file for the first time (i. e. not accessing existing file),\n" +
+                    "to a file for the first time (i. e. not accessing an existing file),\n" +
                     "ChronicleMapBuilder.entries() must be configured.\n" +
                     "See Chronicle Map 3 tutorial and javadocs for more information");
         }