download_strategy: fix detect_from_symbol method (#446)

strategy is an undefined variable

Library/Homebrew/download_strategy.rb

@@ -960,7 +960,7 @@ class DownloadStrategyDetector
     when :post    then CurlPostDownloadStrategy
     when :fossil  then FossilDownloadStrategy
     else
-      raise "Unknown download strategy #{strategy} was requested."
+      raise "Unknown download strategy #{symbol} was requested."
     end
   end
 end