[ci skip] README: amend badge url

README.md

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 # TTG
 This is the C++ API for the Template Task Graph (TTG) programming model for flowgraph-based composition of high-performance algorithms executable on distributed heterogeneous computer platforms. The TTG API abstracts out the details of the underlying task and data flow runtime; the current realization is implemented using [MADNESS](https://github.com/m-a-d-n-e-s-s/madness) and [PaRSEC](https://bitbucket.org/icldistcomp/parsec.git) runtimes as backends.