8381772: Remove mention of "mutable strings" in java.lang.String Javadocs

Reviewed-by: alanb, liach

Author: Eirik Bjørsnøs

src/java.base/share/classes/java/lang/String.java

@@ -70,9 +70,9 @@
  * string literals in Java programs, such as {@code "abc"}, are
  * implemented as instances of this class.
  * <p>
- * Strings are constant; their values cannot be changed after they
- * are created. String buffers support mutable strings.
- * Because String objects are immutable they can be shared. For example:
+ * Strings are immutable; their values cannot be changed after they
+ * are created. Because String objects are immutable they can be shared.
+ * For example:
  * <blockquote><pre>
  *     String str = "abc";
  * </pre></blockquote><p>