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.
```

1 files changed, 1 insertions, 1 deletions

diff --git a/nixos/modules/services/databases/postgresql.nix b/nixos/modules/services/databases/postgresql.nix
index a23a0ebc7366..9b5e3735239f 100644
--- a/nixos/modules/services/databases/postgresql.nix
+++ b/nixos/modules/services/databases/postgresql.nix
@@ -153,7 +153,7 @@ in
         default= if versionAtLeast config.system.stateVersion "17.09" then "postgres" else "root";
         internal = true;
         description = ''
-          NixOS traditionally used `root` as superuser, most other distros use `postgres`.
+          NixOS traditionally used 'root' as superuser, most other distros use 'postgres'.
           From 17.09 we also try to follow this standard. Internal since changing this value
           would lead to breakage while setting up databases.
         '';