docs: avoid users on 3.10 by accident (bazel-contrib#653)

Author: alexeagle

README.md

@@ -60,12 +60,13 @@ To register a hermetic Python toolchain rather than rely on whatever is already
 load("@rules_python//python:repositories.bzl", "python_register_toolchains")
 
 python_register_toolchains(
-    name = "python310",
+    name = "python3_9",
     # Available versions are listed in @rules_python//python:versions.bzl.
-    python_version = "3.10",
+    # We recommend using the same version your team is already standardized on.
+    python_version = "3.9",
 )
 
-load("@python310_resolved_interpreter//:defs.bzl", "interpreter")
+load("@python3_9_resolved_interpreter//:defs.bzl", "interpreter")
 
 load("@rules_python//python:pip.bzl", "pip_parse")