doc: Fix v8 doc headings so 'make doc' passes

Previously the code that builds the Table of Contents threw
an exception because of jump from an H1 heading directly to an H3
heading. This happened when running `make doc`.

By changing the H3 heading to an H2, 'make doc' works again. This is
also consistent with other docs like console.markdown which use
H2 for method call documentation.