docts: update install.md (#883)

Download the binary for the right OS/Arch combination.

Signed-off-by: Raul Gutierrez Segales <rgs@itevenworks.net>

docs/install.md

@@ -6,8 +6,10 @@ On Linux and macOS you can download as follows:
 
 ```console
 LATEST_VERSION=$(wget -O - "https://api.github.com/repos/open-policy-agent/conftest/releases/latest" | grep '"tag_name":' | sed -E 's/.*"([^"]+)".*/\1/' | cut -c 2-)
-wget "https://github.com/open-policy-agent/conftest/releases/download/v${LATEST_VERSION}/conftest_${LATEST_VERSION}_Linux_x86_64.tar.gz"
-tar xzf conftest_${LATEST_VERSION}_Linux_x86_64.tar.gz
+ARCH=$(arch)
+SYSTEM=$(uname)
+wget "https://github.com/open-policy-agent/conftest/releases/download/v${LATEST_VERSION}/conftest_${LATEST_VERSION}_${SYSTEM}_${ARCH}.tar.gz"
+tar xzf conftest_${LATEST_VERSION}_${SYSTEM}_${ARCH}.tar.gz
 sudo mv conftest /usr/local/bin
 ```