Update pdo-firebird.xml Fix typo (#4293)

Author: mmalferov

reference/pdo_firebird/pdo-firebird.xml

@@ -143,7 +143,7 @@
      <listitem>
       <simpara>
        Flag denoting the <acronym>ANSI</acronym> transaction isolation
-       level is read commited.
+       level is read committed.
        This is the default behavior.
       </simpara>
      </listitem>