docs: Added an example for promptChoiceOnce

Author: N3WK1D

assets/chezmoi.io/docs/reference/templates/init-functions/promptChoiceOnce.md

@@ -3,3 +3,12 @@
 `promptChoiceOnce` returns the value of *map* at *path* if it exists and is a
 string, otherwise it prompts the user for one of *choices* with *prompt* and an
 optional *default* using `promptChoice`.
+
+!!! example
+
+    ```
+    {{- $choices := list "desktop" "laptop" "server" "termux" -}}
+    {{- $hosttype := promptChoiceOnce . "hosttype" "What type of host are you on" $choices -}}
+    [data]
+        hosttype = {{- $hosttype | quote -}}
+    ```