Doc: add a missing period (GH-21819)

diff --git a/Doc/library/json.rst b/Doc/library/json.rst
index b923c8e..e1a246a 100644
--- a/Doc/library/json.rst
+++ b/Doc/library/json.rst
@@ -745,7 +745,7 @@
 
 .. cmdoption:: --indent, --tab, --no-indent, --compact
 
-   Mutually exclusive options for whitespace control
+   Mutually exclusive options for whitespace control.
 
    .. versionadded:: 3.9