nixos/doc/manual/development/writing-documentation.xml: fix build command (#262044)

Changed files
+1 -1
nixos
doc
+1 -1
nixos/doc/manual/development/writing-documentation.chapter.md
···
```ShellSession
$ cd /path/to/nixpkgs/nixos/doc/manual
$ nix-shell
-
nix-shell$ make
+
nix-shell$ devmode
```
Once you are done making modifications to the manual, it's important to