update repo name in installation instructions

[BenH11235]

Attempting to run git clone https://github.com/VirusTotal/vt_graph_api on my machine (Ubuntu 20.04.5 LTS, git version 2.39.0) produces the error

remote: Repository not found.
fatal: repository 'https://github.com/VirusTotal/vt_graph_api/' not found

The issue is fixed if vt-graph-api is used instead.

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