Fix GH-1433: SplFixedArray changelog states IteratorAggregate happened in 8.1

Author: cmb69

reference/spl/splfixedarray.xml

@@ -76,9 +76,15 @@
        <entry>8.1.0</entry>
        <entry>
         <classname>SplFixedArray</classname> implements
-        <interfacename>JsonSerializable</interfacename> and
+        <interfacename>JsonSerializable</interfacename> now.
+       </entry>
+      </row>
+      <row>
+       <entry>8.0.0</entry>
+       <entry>
+        <classname>SplFixedArray</classname> implements
         <interfacename>IteratorAggregate</interfacename> now.
-        Previously, only <interfacename>Iterator</interfacename> was implemented instead.
+        Previously, <interfacename>Iterator</interfacename> was implemented instead.
        </entry>
       </row>
      </tbody>