Doc: add a missing period (GH-21819) (#21932)

(cherry picked from commit 12695f4)

Co-authored-by: Mathieu Dupuy <[email protected]>

Co-authored-by: Mathieu Dupuy <[email protected]>

Author: miss-islington

Doc/library/json.rst

@@ -745,7 +745,7 @@ Command line options
 
 .. cmdoption:: --indent, --tab, --no-indent, --compact
 
-   Mutually exclusive options for whitespace control
+   Mutually exclusive options for whitespace control.
 
    .. versionadded:: 3.9