Doc: fix typo in release notes.

Ioseph Kim

Discussion: https://postgr.es/m/e7a79f91-8244-5bcb-afcc-96c817e86f4e@postgresql.kr

Author: tglsfdc

doc/src/sgml/release-10.sgml

@@ -1946,7 +1946,7 @@
 -->
       <para>
        Add function <link
-       linkend="functions-txid-snapshot"><function>txid_current_ifassigned()</></>
+       linkend="functions-txid-snapshot"><function>txid_current_if_assigned()</></>
        to return the current transaction ID or <literal>NULL</> if no
        transaction ID has been assigned (Craig Ringer)
       </para>