fix windows default prompt slash direction (#10555)

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# Description

This fixes the default prompt on Windows to use the correct path
direction

# User-Facing Changes
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helps us keep track of breaking changes. -->

# Tests + Formatting
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crates/nu-utils/src/sample_config/default_env.nu

@@ -25,7 +25,7 @@ def create_left_prompt [] {
     let separator_color = (if (is-admin) { ansi light_red_bold } else { ansi light_green_bold })
     let path_segment = $"($path_color)($dir)"
 
-    $path_segment | str replace --all (char path_sep) $"($separator_color)/($path_color)"
+    $path_segment | str replace --all (char path_sep) $"($separator_color)(char path_sep)($path_color)"
 }
 
 def create_right_prompt [] {