Add theme_display_nix variable to other places.

README.md

@@ -82,6 +82,7 @@ set -g theme_display_docker_machine no
 set -g theme_display_k8s_context yes
 set -g theme_display_hg yes
 set -g theme_display_virtualenv no
+set -g theme_display_nix no
 set -g theme_display_ruby no
 set -g theme_display_nvm yes
 set -g theme_display_user ssh

fish_prompt.fish

@@ -31,6 +31,7 @@
 #     set -g theme_display_k8s_namespace no
 #     set -g theme_display_hg yes
 #     set -g theme_display_virtualenv no
+#     set -g theme_display_nix no
 #     set -g theme_display_ruby no
 #     set -g theme_display_user ssh
 #     set -g theme_display_hostname ssh