[PATCH/obv] doc: Fix substitute-path example

[Eli Zaretskii]

> From: Simon Marchi <simon.marchi@ericsson.com>
> CC: Simon Marchi <simon.marchi@ericsson.com>
> Date: Tue, 22 Sep 2015 16:53:57 -0400
> 
> FYI.
> 
> Found by Marc Khouzam.
> 
> gdb/doc/ChangeLog:
> 
> 	* gdb.texinfo (Specifying Source Directories): Fix
> 	substitute-path example.

Obviously OK.

Thanks.