Fix: Convert help example results to text (#12078)

# Description

Converts help example results `to text` in `build-command-page`. This
prevents an `item_not_found` error when attempting to `help <command>`
on many legitimate commands.

Fixes #12073

# User-Facing Changes

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crates/nu-std/std/help.nu

@@ -647,6 +647,7 @@ def build-command-page [command: record] {
             (if not ($example.result | is-empty) {
                 $example.result
                 | table
+                | to text
                 | if ($example.result | describe) == "binary" { str join } else { lines }
                 | each {|line|
                     $"  ($line)"