Update README.md

fixed typo in README

README.md

diff --git a/README.md b/README.md
index 5d63987..48fe5b5 100644
--- a/README.md
+++ b/README.md
@@ -133,7 +133,7 @@
 
 ```bash
 @test 'assert_dir_exists()' {
-  assert_file_exists /path/to/non-existent-directory
+  assert_dir_exists /path/to/non-existent-directory
 }
 ```