services.postgres: fix quoting in the description of `superUser`

`nixos-option` evals the description and the '`' is used to
define shell commands.

Due to this, the following error appears:

```
$ nixos-option services.postgresql.superUser
Value:
"root"

Default:
"root"

Description:

/run/current-system/sw/bin/nixos-option: line 294: root: command not found
/run/current-system/sw/bin/nixos-option: line 294: postgres: command not found
NixOS traditionally used  as superuser, most other distros use .
From 17.09 we also try to follow this standard. Internal since changing this value
would lead to breakage while setting up databases.
```