util/get_maintainer.pl: Fix top_of_kernel_tree check

The script checks for a folder called documentation, while the folder
name is Documentation.

Without this change, I get this when running the script:
util/scripts/get_maintainer.pl: The current directory does not appear to be
 a coreboot source tree.

Change-Id: Ied7ead7dfec33e9324f06d0ef60dcd6d7ba40104
Signed-off-by: Ben Gardner <gardner.ben@gmail.com>
Reviewed-on: https://review.coreboot.org/13062
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

util/scripts/get_maintainer.pl

@@ -823,7 +823,7 @@ sub top_of_kernel_tree {
     }
     if (   (-f "${lk_path}MAINTAINERS")
 	&& (-f "${lk_path}Makefile")
-	&& (-d "${lk_path}documentation")
+	&& (-d "${lk_path}Documentation")
 	&& (-d "${lk_path}src")
 	&& (-d "${lk_path}util")) {
 	return 1;