installation.md is correct instructions

[Firefishy]

The releases use .tar.gz files while the instructions in installation.md have eg: https://github.com/abtreece/confd/releases/download/v0.18.0/confd-0.18.0-linux-amd64

Dockerfile example:

ARG CONFD_VERSION=0.19.1
RUN CONFD_ARCH=$(arch | sed s/aarch64/arm64/ | sed s/x86_64/amd64/) \
    && curl -SL "https://github.com/abtreece/confd/releases/download/v${CONFD_VERSION}/confd-v${CONFD_VERSION}-linux-${CONFD_ARCH}.tar.gz" | tar -xz -C /usr/local/bin/ \
    && /usr/local/bin/confd -version