Use root README.md and not the link

Author: TTWNO

plotters/Cargo.toml

@@ -9,7 +9,7 @@ repository = "https://github.com/plotters-rs/plotters"
 homepage = "https://plotters-rs.github.io/"
 keywords = ["WebAssembly", "Visualization", "Plotting", "Drawing"]
 categories = ["visualization", "wasm"]
-readme = "README.md"
+readme = "../README.md"
 exclude = ["doc-template", "plotters-doc-data"]
 
 [dependencies]