doc: replace FTP URL to kernel.org with HTTPS one

FTP services were shutdown some weeks ago, so the FTP URL
does not work anymore. Fix this by replacing it with
corresponding HTTPS URL.
Signed-off-by: NMichael Heimpold <michael.heimpold@i2se.com>
Signed-off-by: NJonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/README.rst

@@ -362,7 +362,7 @@ If something goes wrong
    as is, otherwise you will have to use the ``ksymoops`` program to make
    sense of the dump (but compiling with CONFIG_KALLSYMS is usually preferred).
    This utility can be downloaded from
-   ftp://ftp.<country>.kernel.org/pub/linux/utils/kernel/ksymoops/ .
+   https://www.kernel.org/pub/linux/utils/kernel/ksymoops/ .
    Alternatively, you can do the dump lookup by hand:
 
  - In debugging dumps like the above, it helps enormously if you can