update emscripten/project/purr-data/README.md

Zack Leerequested to merge
cuinjune/purr-data:emscripten26 into emscripten

Using the previous Emscripten installation method, I was no longer able to build Purr Data for some reason.

So I had to add the following and now it builds fine.

./emsdk update-tags
./emsdk install tot
./emsdk activate tot

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