mention `fly` alias in flyctl install doc.

[catflydotio]

Our scripts add a system symlink from flyctl to fly. See #555

[ferringb]

Just confirming, it's desired that the docs refer to flyctl rather than fly ?

If so, this bit of horribleness (and manual review) does the bulk of it:

flyctl -h | sed -ne '0,/Deploy/d' -e '/^  .*/p' -e '/Flags/q' | \
  awk '{print "s/fly "$1" /flyctl "$1" /g";}' \
  > patterns
find . -type f \! -wholename '*/.git/*' -print0 | \
  xargs -0 -n1 -P10  sed -f patterns -i
rm patterns

Roughly:

• extract the flyctl subcommands
• build sed patterns to do s/fly <subcommand> /flyctl <subcommand> /g
• run it.