Updates to profiling section

Firstly, some minor corrections and additions to the Java profiling section. The corrections mainly concern a couple of typos that crept into the variable names of the examples. Readers should be reassured that the code, like that of the site in general, is copied and pasted from working, live profiling code. But things such as variable names are occasionally changed or shortened for the purposes of making it clearer on the site, and that seems to be where the errors crept in. I've also taken the opportunity to add a few links to other sections of the site (such as the section on threading, sleep() and yield()) that were added since the profiling tutorial was written.

Readers interested in Java profiling may also be interested in the first page of an upcoming section on Java and memory. This first page looks at how to find out the memory usage of a Java object. The technique involves using the Java Instrumentation framework introduced in version 5 of the language to query the JVM directly for the size of an object. Although slightly fiddly to set up, the technique has the advantage that there's less guesswork involved than if we were to just estimate an object's size (although future pages in the section will nonetheless look at estimation).