Be consistent in the spelling of thread-safe(ty).
diff --git a/Doc/library/runpy.rst b/Doc/library/runpy.rst
index 71f896f..896b65d 100644
--- a/Doc/library/runpy.rst
+++ b/Doc/library/runpy.rst
@@ -120,7 +120,7 @@
Note that, unlike :func:`run_module`, the alterations made to :mod:`sys`
are not optional in this function as these adjustments are essential to
- allowing the execution of sys.path entries. As the thread safety
+ allowing the execution of sys.path entries. As the thread-safety
limitations still apply, use of this function in threaded code should be
either serialised with the import lock or delegated to a separate process.