Fix typo in README.md
Updates "convience" to "convenience" in Toolchain override shorthand section of README.md
@@ -282,7 +282,7 @@ Toolchain names that don't name a channel instead can be used to name
## Toolchain override shorthand
The `rustup` toolchain proxies can be instructed directly to use a
-specific toolchain, a convience for developers who often test
+specific toolchain, a convenience for developers who often test
different toolchains. If the first argument to `cargo`, `rustc` or
other tools in the toolchain begins with `+`, it will be interpreted
as a rustup toolchain name, and that toolchain will be preferred,
