Fix reference to jsonld instead of rdf

Julien LepillerWed May 27 18:35:04+0200 2020

749bb32

Fix reference to jsonld instead of rdf

README.md

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guix install guile guile-rdf
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```
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Otherwise, your package manager might have guile-jsonld available.  You can also
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Otherwise, your package manager might have guile-rdf available.  You can also
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build it from source, like so:
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```bash