Proper coloring for Rust example code

[schmidsi]

https://thegraph.com/docs/en/cookbook/substreams-powered-subgraphs/

Not annotated

[dimaMachina]

Enabling rule to enforce code blocks have language will avoid it in the future

[benface]

@B2o5T – You worked on this in #358, right?

[dimaMachina]

@benface Yes, I already worked on adding this rule, but my pr didn’t receive approvals