use h2 for top-level header

otherwise javadoc complains:

```
[ERROR] /tmp/tmp3xdp27n9/4c8eec7e8183f75869e1154d4ed490149e848e80/core/src/main/java/com/datastax/oss/driver/api/core/paging/OffsetPager.java:47: error: headi
ng used out of sequence: <H3>, compared to implicit preceding heading: <H1>
[ERROR]  * <h3>Performance considerations</h3>
[ERROR]    ^
```

Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>

core/src/main/java/com/datastax/oss/driver/api/core/paging/OffsetPager.java

@@ -46,7 +46,7 @@
  * a reasonable trade-off if the cardinality stays low. This class provides a way to emulate this
  * behavior on the client side.
  *
- * <h3>Performance considerations</h3>
+ * <h2>Performance considerations</h2>
  *
  * For each page that you want to retrieve:
  *
@@ -71,7 +71,7 @@
  * OffsetPager.Page&lt;Row&gt; page5 = pager.getPage(rs, 5);
  * </pre>
  *
- * <h3>Establishing application-level guardrails</h3>
+ * <h2>Establishing application-level guardrails</h2>
  *
  * Linear performance should be fine for the values typically encountered in real-world
  * applications: for example, if the page size is 25 and users never go past page 10, the worst case