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.