doc: update fs callback example error stack

Update the error stack printed in the default callback
example in the fs doc, matching the latest lib/fs.js.
Truncates the stack to make it easier to update in the future.

PR-URL: #6617
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/fs.md

@@ -82,16 +82,11 @@ function bad() {
 bad();
 
 $ env NODE_DEBUG=fs node script.js
-fs.js:66
- throw err;
- ^
-Error: EISDIR, read
- at rethrow (fs.js:61:21)
- at maybeCallback (fs.js:79:42)
- at Object.fs.readFile (fs.js:153:18)
- at bad (/path/to/script.js:2:17)
- at Object.<anonymous> (/path/to/script.js:5:1)
- <etc.>
+fs.js:88
+ throw backtrace;
+ ^
+Error: EISDIR: illegal operation on a directory, read
+ <stack trace.>
 ```
 
 ## Buffer API