Fix broken link - Technical Reference

[orisdorch]

The intended PDF does not exist at https://docs.symbol.dev/symbol-technicalref/main.pdf as presently linked. The correct source file appears to be https://symbol.github.io/symbol-technicalref/main.pdf as per https://github.com/symbol/symbol-technicalref

Alternate approach would be to add a copy of the technical reference at https://docs.symbol.dev/symbol-technicalref/main.pdf

Loosely relates to #839

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