docs: Fix default options for fs.createWriteStream()

The documentation for createWriteStream() references an 'encoding' property that has a default value of null. However, this property is never referenced by createWriteStream() or WritableState(). Instead a 'defaultEncoding' property is referenced in WritableState() with a default of 'utf8' if no value is supplied. This fix updates the documentation to rename the 'encoding' property to 'defaultEncoding' and indicate its default value of 'utf8'. Reviewed-By: James M Snell <[email protected]> PR-URL: nodejs/node-v0.x-archive#25591

Author: chrisneave

doc/api/fs.markdown

@@ -823,7 +823,7 @@ Returns a new WriteStream object (See `Writable Stream`).
 `options` is an object with the following defaults:
 
 {flags: 'w',
-encoding: null,
+defaultEncoding: 'utf8',
  fd: null,
  mode: 0666 }