Update VB Syntax highlighting so Github understands it

**NOTE**: Since this is a minor change to Markdown files, I'm assuming it doesn't require a CLA, so I figured I would just submit a PR instead of going through the rigamarole just yet. :)

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GitHub's markdown syntax highlighting doesn't appear to recognize `VisualBasic`, which is how VB code sample languages are specified in all the markdown files.

I experimented and noticed that changing the language in code samples to `VB.net` corrected the syntax highlighting.

I did a replace across all files, changing "VisualBasic" to "VB.net" and the syntax highlighting for Visual basic appears to show correctly now.

I know it's a larger pull request (sorry!) but since it was a simple find/replace and I could review it, I decided to handle it this way.