Swap wget for curl to be consistent with docs

[ipedrazas]

The quickstart has install instructions using curl, however, the install script is using wget.

Quickstart:

curl https://spin.fermyon.dev/downloads/install.sh | bash

Install script:

# Download file, exit if not found - e.g. version does not exist
fancy_print 0 "Step 1: Downloading: ${URL}"
wget -q $URL || (fancy_print 1 "The requested file does not exist: ${FILE}"; exit 1)
fancy_print 0 "Done...\n"

This PR replaces the wget instructions for the equivalent curl ones, to keep the tool used in the documentation and in the script consistent.

[mikkelhegn]

Cool - thx!