Fixed README path in pyo3-asyncio manifest

Author: Andrew J Westlake

Cargo.toml

@@ -3,7 +3,7 @@ name = "pyo3-asyncio"
 description = "PyO3 utilities for Python's Asyncio library"
 version = "0.13.0"
 authors = ["Andrew J Westlake <[email protected]>"]
-readme = "/README.md"
+readme = "README.md"
 keywords = ["pyo3", "python", "ffi", "async", "asyncio"]
 homepage = "https://github.com/awestlake87/pyo3-asyncio"
 repository = "https://github.com/awestlake87/pyo3-asyncio"