[ci skip] Fix indentation in `messagebuilder.md`

Created by: billyeatcookies

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Summary

• Fixes indentation in docs/articles/beyond_basics/messagebuilder.md

Details

Code provided under Adding a File section is indented wrong, this pull request corrects it.

Changes proposed

Before:

 using (var fs = new FileStream("ADumbFile.txt", FileMode.Open, FileAccess.Read))
 {
    var msg = await new DiscordMessageBuilder()
        .WithContent("Here is a really dumb file that I am testing with.")
        .WithFiles(new Dictionary<string, Stream>() { { "ADumbFile1.txt", fs } })
        .SendAsync(ctx.Channel);           
}

After:

using (var fs = new FileStream("ADumbFile.txt", FileMode.Open, FileAccess.Read))
{
    var msg = await new DiscordMessageBuilder()
        .WithContent("Here is a really dumb file that I am testing with.")
        .WithFiles(new Dictionary<string, Stream>() { { "ADumbFile1.txt", fs } })
        .SendAsync(ctx.Channel);           
}