Update lib/rack/response.rb

Co-authored-by: Jeremy Evans <code@jeremyevans.net>

lib/rack/response.rb

@@ -41,7 +41,7 @@ def header
     # Initialize the response object with the specified body, status
     # and headers.
     #
-    # @param body [nil, #each, #call, #to_s] the response body.
+    # @param body [nil, #each, #call, #to_str] the response body.
     # @param status [Integer] the integer status as defined by the
     # HTTP protocol RFCs.
     # @param headers [Hash] a hash of key-value header pairs which