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.

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.

PR-URL: #203
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>

doc/api/v8.markdown

@@ -6,7 +6,7 @@ This module exposes events and interfaces specific to the version of [V8][]
 built with node.  These interfaces are subject to change by upstream and are
 therefore not covered under the stability index.
 
-### getHeapStatistics()
+## getHeapStatistics()
 
 Returns an object with the following properties
 
@@ -20,7 +20,7 @@ Returns an object with the following properties
 }
 ```
 
-### setFlagsFromString()
+## setFlagsFromString()
 
 Set additional V8 command line flags.  Use with care; changing settings
 after the VM has started may result in unpredictable behavior, including