docs: correct typo-ed path to heapam_handler.c.

Reported-By: Michael Paquier, Michel Pelletier
Discussion:
   https://postgr.es/m/20190410025531.GA2728@paquier.xyz
   https://postgr.es/m/CACxu=v+u_QTeFqdajCHv3i4QmzV_63arVb57R19dSKtThdSLkQ@mail.gmail.com

doc/src/sgml/tableam.sgml

@@ -104,7 +104,7 @@
  <para>
   Any developer of a new <literal>table access method</literal> can refer to
   the existing <literal>heap</literal> implementation present in
-  <filename>src/backend/heap/heapam_handler.c</filename> for details of
+  <filename>src/backend/access/heap/heapam_handler.c</filename> for details of
   its implementation.
  </para>