Fix bug in solution of exercise 3, also update the describe of problem.

exercises/my-first-io/problem/problem.md

@@ -12,6 +12,6 @@ To read a file, you'll need to use `file_get_contents('/path/to/file')`. This me
 Documentation on the `file_get_contents` function can be found by pointing your browser here:
   [http://php.net/manual/en/function.file-get-contents.php]()
 
-If you're looking for an easy way to count the number of newlines in a string, recall the PHP function `explode` can be used to turn a string into an array of substrings and that "\n" can be used as a delimiter.
+If you're looking for an easy way to count the number of newlines in a string, recall the PHP function `substr_count` can be used to count the number of substring occurrences.
 
 ----------------------------------------------------------------------

exercises/my-first-io/solution/solution.php

@@ -1,5 +1,4 @@
 <?php
 
 $contents = file_get_contents($argv[1]);
-$lines = explode("\n", $contents);
-echo count($lines);
+echo substr_count($contents, "\n");