docs: readme: fixed example: fileName -> filename (#269)

README.md

@@ -112,7 +112,7 @@ If you're instrumenting code programatically, you can pass a source map explicit
 ```js
 import babelPluginIstanbul from 'babel-plugin-istanbul';
 
-function instrument(sourceCode, sourceMap, fileName) {
+function instrument(sourceCode, sourceMap, filename) {
   return babel.transform(sourceCode, {
     filename,
     plugins: [