Issue with General format specifier defaults

The table that lists the default precision for the "G" specifier is no longer correct (at least for double) for .NET Core 3 and beyond. For instance, for double, it specifies the default precision is 15, but that's no longer the case in .NET Core 3:

```cs
// .NET Framework
99.199999999999989d.ToString() // -> 99.2
99.199999999999989d.ToString("G15") // -> 99.2

// .NET Core 3
99.199999999999989d.ToString() // -> 99.19999999999999
99.199999999999989d.ToString("G15") // -> 99.2
```
[This blog post](https://devblogs.microsoft.com/dotnet/floating-point-parsing-and-formatting-improvements-in-net-core-3-0/) describes the breaking change, and [this is an example](https://github.com/dotnet/runtime/issues/29157) of an issue that was opened because of the breaking change. The docs confused me for some time until I found the above two links.

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#### Document Details

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* ID: 0ae38fce-fc98-9420-6b0b-ceee4e885f2d
* Version Independent ID: f23035e0-f271-b48c-f9a9-8582be80b4ad
* Content: [Standard numeric format strings](https://docs.microsoft.com/en-us/dotnet/standard/base-types/standard-numeric-format-strings#GFormatString)
* Content Source: [docs/standard/base-types/standard-numeric-format-strings.md](https://github.com/dotnet/docs/blob/main/docs/standard/base-types/standard-numeric-format-strings.md)
* Product: **dotnet-fundamentals**
* GitHub Login: @adegeo
* Microsoft Alias: **adegeo**