ARROW-6705: [Rust] [DataFusion] README has invalid github URL
ARROW-6705: [Rust] [DataFusion] README has invalid github URL

Closes #5499 from alippai/patch-1 and squashes the following commits:

d2fd03e68 <Ádám Lippai> Update README.md

Authored-by: Ádám Lippai <0e18f44c1fec03ec4083422cb58ba6a09ac4fb2a@rigo.sk>
Signed-off-by: Andy Grove <f68909d51919859739a1c5d24450a2cc1026e93b@gmail.com>

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