php(bob): fix "toOrdinal" to "respondTo"

In Bob, the placeholder said "Implement the toOrdinal function" instead of respondTo.
This PR is a very minor one that solves this.

exercises/practice/bob/Bob.php

@@ -28,6 +28,6 @@ class Bob
 {
     public function respondTo(string $str): string
     {
-        throw new \BadMethodCallException("Implement the toOrdinal function");
+        throw new \BadMethodCallException("Implement the respondTo function");
     }
 }