doc: fix use of deprecated config file in the example

doc: fix use of deprecated config file in the example

Commit 31c58c139d85 ("jtag: drivers: stlink: handle all versions with
single config") deprecates the use of "interface/stlink-v2-1.cfg" in
favor of a unique config file "interface/stlink.cfg".

Update the example in the documentation.

Change-Id: I1aed7c70e15f4edb4f81a3ee8e3bce575fde873b
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/4667
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>