suggestions from review

Co-Authored-By: David Hewitt <1939362+davidhewitt@users.noreply.github.com>

guide/src/exception.md

@@ -50,7 +50,7 @@ fn main() {
 }
 ```
 
-From `pyfunction`s and `pyclass` methods, returning a `Err(PyErr)` is enough;
+From `pyfunction`s and `pyclass` methods, returning an `Err(PyErr)` is enough;
 PyO3 will handle restoring the exception on the Python interpreter side.
 
 If you already have a Python exception instance, you can simply call