nixos/postgresql: fix identMap example

This was causing the following warning before when building the manual:

    warning: literalExample is deprecated, use literalExpression instead, or use literalMD for a non-Nix description.

Rather than using `literalExpression`, nothing is used. This option
expects a string and the example is a string, no special handling
required. Both `literalExample` from the docbook ages and
`literalExpression` now are only required if the example is
a Nix expression rather than a value of the option's type.

nixos/modules/services/databases/postgresql.nix

@@ -106,7 +106,7 @@ in
       identMap = mkOption {
         type = types.lines;
         default = "";
-        example = literalExample ''
+        example = ''
           map-name-0 system-username-0 database-username-0
           map-name-1 system-username-1 database-username-1
         '';