Correct spelling: longjump() -> longjmp().

doc/src/sgml/extend.sgml

@@ -322,7 +322,7 @@
        If calling backend functions from C++ code, be sure that the
        C++ call stack contains only plain old data structures
        (<acronym>POD</>).  This is necessary because backend errors
-       generate a distant <function>longjump()</> that does not properly
+       generate a distant <function>longjmp()</> that does not properly
        unroll a C++ call stack with non-POD objects.
       </para>
      </listitem>