brew-update: show upgrade command on formula parameters.

Closes Homebrew/homebrew#17814.

Library/Homebrew/cmd/update.rb

@@ -6,7 +6,12 @@ module Homebrew extend self
   DEPRECATED_TAPS = ['adamv-alt']
 
   def update
-    abort "This command updates brew itself, and does not take formula names." unless ARGV.named.empty?
+    unless ARGV.named.empty?
+      abort <<-EOS.undent
+        This command updates brew itself, and does not take formula names.
+        Use `brew upgrade <formula>`.
+      EOS
+    end
     abort "Please `brew install git' first." unless which "git"
 
     # ensure GIT_CONFIG is unset as we need to operate on .git/config