bpo-40630: adjust tracemalloc.reset_peak docs for backport to 3.9 (GH-20546)

diff --git a/Doc/library/tracemalloc.rst b/Doc/library/tracemalloc.rst
index fba1caa..20f668c 100644
--- a/Doc/library/tracemalloc.rst
+++ b/Doc/library/tracemalloc.rst
@@ -345,7 +345,7 @@
 
    See also :func:`get_traced_memory`.
 
-   .. versionadded:: 3.10
+   .. versionadded:: 3.9
 
 
 .. function:: get_tracemalloc_memory()