scripts/coccicheck: Update reference for the corresponding documentation

Use the current name (in a comment at the beginning of this script) for
the file which was converted to the documentation format "reStructuredText"
in August 2016.

Fixes: 4b9033a3 ("docs: sphinxify coccinelle.txt and add it to dev-tools")
Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Acked-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Michal Marek <mmarek@suse.com>

scripts/coccicheck

 #!/bin/bash
 # Linux kernel coccicheck
 #
-# Read Documentation/coccinelle.txt
+# Read Documentation/dev-tools/coccinelle.rst
 #
 # This script requires at least spatch
 # version 1.0.0-rc11.