Fix incorrect usage of regex in WPF sample

[GrabYourPitchforks]

I'm updating regex docs across the various docs repos. This is one part of that update.

Changes:

• Demonstrates proper use of regex escaping when searching for an input string.
• Demonstrates proper use of word boundaries.
• Corrects a typo.

Related PRs:

• Fix incorrect usage of regex in WPF sample #2250 (this one)
• Reword some regex safety warnings docs#54621
• Reword some regex safety warnings dotnet-api-docs#12848

[Copilot]

Pull request overview

This PR updates the WPF “Find” dialog box samples (C# and Visual Basic) to demonstrate safer, more correct regex usage when searching for user-provided input in the text box being searched.

Changes:

• Escapes user input via Regex.Escape before building the search pattern.
• Replaces the prior whole-word lookaround logic with conditional \b word-boundary usage.
• Fixes a typo in the “No more matches found…” prompt.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
dotnet-desktop-guide/samples/snippets/visualbasic/VS_Snippets_Wpf/DialogBoxSample/VisualBasic/FindDialogBox.xaml.vb	Updates VB sample to escape literal input, apply word boundaries more correctly, and fix UI prompt text.
dotnet-desktop-guide/samples/snippets/csharp/VS_Snippets_Wpf/DialogBoxSample/CSharp/FindDialogBox.xaml.cs	Updates C# sample with the same regex escaping/word-boundary improvements and typo fix.