[Bug] IndexOutOfRangeException with FSharp assembly active pattern

[MadelineRitchie]

Describe the bug
Attempting to generate docs against an fsharp assembly with a particular shape of active pattern throws an exception. I created a minimum reproduction https://github.com/MadelineRitchie/docfx-exception-repro and the code that causes docfx to fail is there in src/lib/Library.fs

To Reproduce
Steps to reproduce the behavior:

1. Clone my minimum repro https://github.com/MadelineRitchie/docfx-exception-repro
2. cd src/docfx
3. docfx.exe .\docfx.json --serve

Expected behavior
Doc output should be generated.

Context (please complete the following information):

• OS: Windows
• Docfx versions tested:

1. 2.77.0+bd00e2b93951e9e7fa6e5abd990d2cd77d7a83bd
2. docfx repo main built locally as of half an hour ago

Additional context
The full console output is in the repro repo at https://github.com/MadelineRitchie/docfx-exception-repro/blob/main/console-output-with-error.txt

I ran docfx with a debugger attached, and this particular FSharp output fails the Debug.Assert in SpecIdHelper.cs at line 49 which currently looks like Debug.Assert(symbol.TypeParameters.Length > int.Parse(match.Value.Substring(2)));