File.LinkError - fix confusing wording

The api for links is ln(existing, new) but the old error text said "from
existing to new", where I believe it should have said "to existing from
new". The language for this has always been a bit confusing IMO, dating
back to the original unix manpages for hard and symbolic links: the
order of the arguments is because of the defaulting rules for the
command line tool, but in a language context without the defaulting
arguably it would be more logical (but against tradition) to reverse the
arguments, but that boat has long ago sailed. However we can at least
make the error message more intuitive.

This patch changes the text to try to make it more clear which is the
new link and which is the thing being linked to. Likely this error came
from the example using "source.txt" and "target.txt" and it being
natural to say "to target", when really in the function call
"target.txt" is the link being created and "source.txt" is the thing
being linked to.

This changes the example to use "existing.txt" and "link.txt" so it is
more obvious which is the link and which is the existing file, and uses
the word "to" in reference the to the thing being linked to, and "from"
in reference to link being created.

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