docs(readme): fix typo

README.md

@@ -29,7 +29,7 @@ $ git clone git@github.com:deltachat/deltachat-ios.git
 $ cd deltachat-ios
 $ git submodule update --init --recursive
 # Make sure the correct rust version is installed
-$ rustup toolchain install `cat deltachat-ios/libraries/deltachat-core-rust/rust-toolchain``
+$ rustup toolchain install `cat deltachat-ios/libraries/deltachat-core-rust/rust-toolchain`
 $ open deltachat-ios.xcworkspace # do not: open deltachat-ios.xcodeproj
 ```