[skip-ci][NFC][ntuple] rename specifications.md --> specification.md

[jblomer]

[pcanal]

Since we are changing the name of the file, this is a good time to ask whether it should be more specific. For example a variation on BinaryFormatSpecification.md to distinguish it from any API/C++ level specification.

[silverweed]

I'm personally fine with specification, as I wouldn't consider our API a "specification" in any sense of the term: just a particular implementation of the binary specs.

[jblomer]

Here is a suggestion with BinaryFormatSpecification.md and, for consistency, Architecture.md (capitalized).

[hahnjo]

I don't have a strong opinion how the files should be named...