scripts: documentation-file-ref-check: Add line break before exit

If execute ./scripts/documentation-file-ref-check in a directory which is
not a git tree, it will exit without a line break, fix it.

Without this patch:

[loongson@localhost linux-5.7-rc1]$ ./scripts/documentation-file-ref-check
Warning: can't check if file exists, as this is not a git tree[loongson@localhost linux-5.7-rc1]$

With this patch:

[loongson@localhost linux-5.7-rc1]$ ./scripts/documentation-file-ref-check
Warning: can't check if file exists, as this is not a git tree
[loongson@localhost linux-5.7-rc1]$
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Reviewed-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/1586857308-2040-1-git-send-email-yangtiezhu@loongson.cnSigned-off-by: Jonathan Corbet <corbet@lwn.net>

scripts/documentation-file-ref-check

@@ -25,7 +25,7 @@ my $fix = 0;
 my $warn = 0;
 
 if (! -d ".git") {
-	printf "Warning: can't check if file exists, as this is not a git tree";
+	printf "Warning: can't check if file exists, as this is not a git tree\n";
 	exit 0;
 }