fix typo

Co-authored-by: Lily Acorn <code@lilyf.org>

Author: Icxolu

guide/src/migration.md

@@ -20,7 +20,7 @@ For this reason we chose to rename these to more modern terminology introduced i
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-The type alias `PyObject` (aka `Py<PyAny>`) is often confused with the identically named FFI definition `pyo3::ffi::PyObject`. For this reason we are deprecating is usage. To migrate simply replace it's usage by the target type `Py<PyAny>`.
+The type alias `PyObject` (aka `Py<PyAny>`) is often confused with the identically named FFI definition `pyo3::ffi::PyObject`. For this reason we are deprecating its usage. To migrate simply replace its usage by the target type `Py<PyAny>`.
 
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