fix s/Long/Fixed in FixedBitSet javadocs (#13290)

(cherry picked from commit f44ded0)

lucene/core/src/java/org/apache/lucene/util/FixedBitSet.java

@@ -116,7 +116,7 @@ public static long andNotCount(FixedBitSet a, FixedBitSet b) {
   }
 
   /**
-   * Creates a new LongBitSet. The internally allocated long array will be exactly the size needed
+   * Creates a new FixedBitSet. The internally allocated long array will be exactly the size needed
    * to accommodate the numBits specified.
    *
    * @param numBits the number of bits needed
@@ -128,7 +128,7 @@ public FixedBitSet(int numBits) {
   }
 
   /**
-   * Creates a new LongBitSet using the provided long[] array as backing store. The storedBits array
+   * Creates a new FixedBitSet using the provided long[] array as backing store. The storedBits array
    * must be large enough to accommodate the numBits specified, but may be larger. In that case the
    * 'extra' or 'ghost' bits must be clear (or they may provoke spurious side-effects)
    *