Replace possessive ASCII apostrophe ('s) with Unicode apostrophe (’s).

Only replaced in end-user messages and documentation, not in code,
comments, or anything else that's not end-user visible.

See: https://www.cl.cam.ac.uk/~mgk25/ucs/quotes.html

generator/java.ml

@@ -189,7 +189,7 @@ public class GuestFS {
    * <code>events</code> is one or more <code>EVENT_*</code> constants,
    * bitwise ORed together.
    * </p><p>
-   * When an event happens, the callback object's <code>event</code> method
+   * When an event happens, the callback object’s <code>event</code> method
    * is invoked like this:
    * </p>
    * <pre>