Fix Cargo.toml repository URL

[karip]

The Cargo.toml file has a wrong URL in its repository field:

repository = "https://github.com/savge13/agg"

It should be:

repository = "https://github.com/savage13/agg"

The result is that e.g. the crates.io page uses the invalid savge13 repository URL (in the panel on the right side): https://crates.io/crates/agg