File.LinkError - fix confusing wording

The api for links is ln(existing, new) but the old text said
"from existing to new", where it should have said "to existing from new".

This fixes the text to make it more clear. Likely this error
came from the example using "source.txt" and "target.txt" and
it being natural to say "to target".

This changes the example to use "existing.txt" and "link.txt"
so it is more obvious what is going on.

Author: Yves Orton

lib/elixir/lib/exception.ex

@@ -2492,8 +2492,8 @@ defmodule File.LinkError do
 
   For example, this exception is raised when trying to link to file that isn't present:
 
-      iex> File.ln!("source.txt", "target.txt")
-      ** (File.LinkError) could not create hard link from "source.txt" to "target.txt": no such file or directory
+      iex> File.ln!("existing.txt", "link.txt")
+      ** (File.LinkError) could not create hard link to "existing.txt" from "link.txt": no such file or directory
 
   The following fields of this exception are public and can be accessed freely:
 
@@ -2509,7 +2509,7 @@ defmodule File.LinkError do
   def message(exception) do
     formatted = IO.iodata_to_binary(:file.format_error(exception.reason))
 
-    "could not #{exception.action} from #{inspect(exception.existing)} to " <>
+    "could not #{exception.action} to #{inspect(exception.existing)} from " <>
       "#{inspect(exception.new)}: #{formatted}"
   end
 end