fix: replaces canvas-prebuilt by canvas

[cjuenger]

When I tried to to install the milsymbol-server I got the following issues:

npm ERR! code 1
npm ERR! path MY-PATH\github\milsymbol-server\node_modules\canvas-prebuilt
npm ERR! command failed
npm ERR! command MY-PATH\cmd.exe /d /s /c node_modules/.bin/node-pre-gyp install
npm ERR! Der Befehl "node_modules" ist entweder falsch geschrieben oder
npm ERR! konnte nicht gefunden werden.

I solved it by replacing canvas-prebuilt by canvas.