Update README.md

Update an example to be inexplicably correct (it's the characters needed for parsing JSON).
diff --git a/README.md b/README.md
index 856d5f3..9c2aaa2 100644
--- a/README.md
+++ b/README.md
@@ -27,7 +27,7 @@
 
 Example:
 ```bash
-dart run charcode -o lib/src/charcodes.dart "09ftn{}[],:"
+dart run charcode -o lib/src/charcodes.dart "09.e\-ftn{}[],:"
 ```
 
 Run `dart run charcode --help` to see other options.