Update README.md

Changed 'writeReport' to 'write' as shown here: http://gotwarlost.github.io/istanbul/public/apidocs/classes/Reporter.html. 'writeReport' is a method of Istanbul's 'Report' class and 'write' is the corresponding method of 'Reporter'.

Great module by the way.
Merge pull request #235 from wlabranche/patch-1

Update README.md