Update external_completers.md (nushell#1883)

Invert fish quotes to prevent issues where the output is a single double quote, breaking the string substitution

nushell#1881

Author: Beinsezii

cookbook/external_completers.md

@@ -20,7 +20,7 @@ This completer will use [the fish shell](https://fishshell.com/) to handle compl
 
 ```nu
 let fish_completer = {|spans|
-    fish --command $'complete "--do-complete=($spans | str join " ")"'
+    fish --command $"complete '--do-complete=($spans | str join ' ')'"
     | from tsv --flexible --noheaders --no-infer
     | rename value description
     | update value {