File.LinkError - swap order of new and existing in error message

The api for creating links is ln(existing, new) but the old error text
said "from existing to new", where I believe it should say "from new to
existing".

Likely this error came from the example using "source.txt" and
"target.txt" and it being natural to say "to target", so this patch also
changes the example to use "existing.txt" and "link.txt" as the file
names so it reads more naturally.

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 from "link.txt" to "existing.txt": no such file or directory
 
   The following fields of this exception are public and can be accessed freely:
 
@@ -2509,8 +2509,8 @@ 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 " <>
-      "#{inspect(exception.new)}: #{formatted}"
+    "could not #{exception.action} from #{inspect(exception.new)} to " <>
+      "#{inspect(exception.existing)}: #{formatted}"
   end
 end