Make path to pg_service.conf absolute in documentation

The system file is always in the absolute path /etc/, not relative.

David Fetter

doc/src/sgml/libpq.sgml

@@ -6843,7 +6843,7 @@ myEventProc(PGEventId evtId, void *evtInfo, void *passThrough)
    at <filename>~/.pg_service.conf</filename> or the location
    specified by the environment variable <envar>PGSERVICEFILE</envar>,
    or it can be a system-wide file
-   at <filename>etc/pg_service.conf</filename> or in the directory
+   at <filename>/etc/pg_service.conf</filename> or in the directory
    specified by the environment variable
    <envar>PGSYSCONFDIR</envar>.  If service definitions with the same
    name exist in the user and the system file, the user file takes