Update ob-flush.xml (#3232)

Changed "it's" to "its" for a possessive, not contraction.

Author: abrothman-vacatia

reference/outcontrol/functions/ob-flush.xml

@@ -15,7 +15,7 @@
   <para>
    This function calls the output handler
    (with the <constant>PHP_OUTPUT_HANDLER_FLUSH</constant> flag),
-   flushes (sends) it's return value
+   flushes (sends) its return value
    and discards the contents of the active output buffer.
   </para>
   <para>