Remove keyword from manifest

We currently have six keywords but only five are allowed, remove the
"bech32m" (we have "bech32" also).

Author: tcharding

Cargo.toml

@@ -6,7 +6,7 @@ repository = "https://github.com/rust-bitcoin/rust-bech32"
 documentation = "https://docs.rs/bech32/"
 description = "Encodes and decodes the Bech32 format and implements the bech32 and bech32m checksums"
 readme = "README.md"
-keywords = ["base32", "encoding", "bech32", "bech32m", "bitcoin", "cryptocurrency"]
+keywords = ["base32", "encoding", "bech32", "bitcoin", "cryptocurrency"]
 categories = ["encoding", "cryptography::cryptocurrencies"]
 license = "MIT"
 edition = "2018"