Make error message clearer

Co-authored-by: Bo Anderson <mail@boanderson.me>

Library/Homebrew/formula.rb

@@ -1616,7 +1616,7 @@ class Formula
   sig { params(source: Pathname, target: Pathname).returns(String) }
   def rpath(source: bin, target: lib)
     unless target.to_s.start_with?(HOMEBREW_PREFIX)
-      raise "`target` should only be used for paths inside HOMEBREW_PREFIX!"
+      raise "rpath `target` should only be used for paths inside HOMEBREW_PREFIX!"
     end
 
     "#{loader_path}/#{target.relative_path_from(source)}"