org-manual: Use present tense if possible

* doc/org-manual.org (Repeated tasks): Prefer present tense over
future tense whenever applicable.

doc/org-manual.org

@@ -6336,7 +6336,7 @@ special repeaters =++= and =.+=.  For example:
 
 ,** TODO Check the batteries in the smoke detectors
    DEADLINE: <2005-11-01 Tue .+1m>
-   Marking this DONE will shift the date to one month after today.
+   Marking this DONE shifts the date to one month after today.
 
 ,** TODO Wash my hands
    DEADLINE: <2019-04-05 08:00 Sun .+1h>