Fix method name to 'toDateString()' in Date and Times user guide

Author: musmanikram

user_guide_src/source/libraries/time.rst

@@ -177,7 +177,7 @@ toDateString()
 Displays just the date portion of the Time::
 
     $time = Time::parse('March 9, 2016 12:00:00', 'America/Chicago');
-    echo $time->toDateTimeString();     // 2016-03-09
+    echo $time->toDateString();     // 2016-03-09
 
 toTimeString()
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