Remove duplicated keywword

crates.io does not accept more than 5 keywords anymore, which caused my
release to fail:

	error: failed to publish to registry at https://crates.io

	Caused by:
	  the remote server responded with an error: invalid upload request: invalid length 6, expected at most 5 keywords per crate at line 1 column 2001

Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 name = "rb"
 description = "A thread-safe ring buffer with blocking IO"
-keywords = ["IO", "audio", "ring-buffer", "ringbuffer", "circular-buffer", "circularbuffer"]
+keywords = ["IO", "audio", "ring-buffer", "circular-buffer"]
 version = "0.3.2"
 authors = ["Andreas Linz <[email protected]>"]
 documentation = "https://docs.rs/rb"