Fix typo in assert.xml (#4691)

Author: msamesch

reference/info/functions/assert.xml

@@ -91,7 +91,7 @@
        <entry>
         <para>
          A user defined function to call when an assertion fails.
-         It's signature should be:
+         Its signature should be:
          <methodsynopsis role="procedural">
           <type>void</type><methodname>assert_callback</methodname>
           <methodparam><type>string</type><parameter>file</parameter></methodparam>