Updated links to VTK specification

[williamjsdavis]

Links to the VTK specification point to a page that does not exist any more:

http://www.vtk.org/VTK/img/file-formats.pdf

These links appear in:

1. The README (

   WriteVTK.jl/README.md

   Line 63 in d15119c

   the [VTK specification](http://www.vtk.org/VTK/img/file-formats.pdf).

   )
2. The documentation index page (

   WriteVTK.jl/docs/src/index.md

   Line 56 in d15119c

   the [VTK specification](http://www.vtk.org/VTK/img/file-formats.pdf).

   ), and
3. The documentation for unstructured grids (

   WriteVTK.jl/docs/src/grids/unstructured.md

   Line 49 in d15119c

   [VTK specification](http://www.vtk.org/VTK/img/file-formats.pdf) (figures 2 and 3).

   )

I have replaced these links with the new appropriate link to the VTK specification in the documentation:

https://docs.vtk.org/en/latest/design_documents/VTKFileFormats.html

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.87%. Comparing base (d15119c) to head (a6b28bd).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #145   +/-   ##
=======================================
  Coverage   96.87%   96.87%           
=======================================
  Files          15       15           
  Lines         863      863           
=======================================
  Hits          836      836           
  Misses         27       27           

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[jipolanco]

Thank you!