bpo-39573: Fix bad copy-paste in Py_SET_SIZE (GH-18496)

diff --git a/Doc/c-api/structures.rst b/Doc/c-api/structures.rst
index 75e2383..083c374 100644
--- a/Doc/c-api/structures.rst
+++ b/Doc/c-api/structures.rst
@@ -103,7 +103,7 @@
 
 .. c:function:: void Py_SET_SIZE(PyVarObject *o, Py_ssize_t size)
 
-   Set the object *o* size of *size*.
+   Set the object *o* size to *size*.
 
    .. versionadded:: 3.9