Fix README.md path in Cargo.toml (#145)

Signed-off-by: Gabriel Féron <gabriel.feron@here.com>

Author: gferon

flatdata-rs/lib/Cargo.toml

@@ -7,7 +7,7 @@ description = "Rust implementation of flatdata"
 repository = "https://github.com/heremaps/flatdata"
 keywords = ["serialization", "flatdata", "mmap"]
 categories = ["encoding"]
-readme = "README.md"
+readme = "../README.md"
 edition = "2018"
 
 [dependencies]