fs_mitm/fs_dir_utils: Amend logic error within IterateDirectoryRecursivelyInternal() (#498)

This likely intended to get the size of the string and then subtract the
null terminator byte. Instead, this would always result in a sizeof of
8.

stratosphere/ams_mitm/source/fs_mitm/fs_dir_utils.hpp

@@ -32,7 +32,7 @@ class FsDirUtils {
                 return rc;
             }
 
-            const size_t parent_len = strnlen(work_path.str, sizeof(work_path.str - 1));
+            const size_t parent_len = strnlen(work_path.str, sizeof(work_path.str) - 1);
 
             /* Read and handle entries. */
             while (true) {