documentation: Remove brackets from README.md

[rbasso]

Removes brackets accidentally inserted in README.md by #374.

[rbasso]

Travis failed...

$ travis_retry curl -L https://www.stackage.org/stack/linux-x86_64 -o pack.tgz
$ tar xzf pack.tgz --wildcards --strip-components=1 -C ${HOME}/bin '/stack'
tar (child): pack.tgz: Cannot open: No such file or directory
tar (child): Error is not recoverable: exiting now
tar: Child returned status 2
tar: Error is not recoverable: exiting now
The command "tar xzf pack.tgz --wildcards --strip-components=1 -C ${HOME}/bin '/stack'" failed and exited with 2 during .

This seems to happen from time to time. I hope that we'll be able to install it using apt with a mirror soon. travis-ci/apt-package-safelist#379

[petertseng]

there had been 503 of that curl at the time of failing. Guess it is OK now.