fix: correct readme field in Cargo.toml to use string format (apache#17310)

Author: Weijun-H

datafusion/spark/Cargo.toml

@@ -22,7 +22,7 @@ version = { workspace = true }
 homepage = { workspace = true }
 repository = { workspace = true }
 authors = { workspace = true }
-readme = { workspace = true }
+readme = "README.md"
 license = { workspace = true }
 edition = { workspace = true }