docs: fix repo ref

Author: robjtede

Cargo.toml

@@ -4,7 +4,7 @@ version = "0.6.3"
 description = "Design-by-contract attributes"
 authors = ["karroffel <[email protected]>"]
 edition = "2018"
-repository = "https://gitlab.com/x52dev/contracts"
+repository = "https://github.com/x52dev/contracts"
 license = "MPL-2.0"
 categories = ["development-tools", "development-tools::procedural-macro-helpers"]
 keywords = ["design-by-contract", "precondition", "postcondition", "invariant", "verification"]