docs: fix locations of several documents that got moved

The previous patch renamed several files that are cross-referenced
along the Kernel documentation. Adjust the links to point to
the right places.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
diff --git a/Documentation/blockdev/ramdisk.txt b/Documentation/blockdev/ramdisk.txt
index fe2ef97..501e12e 100644
--- a/Documentation/blockdev/ramdisk.txt
+++ b/Documentation/blockdev/ramdisk.txt
@@ -14,7 +14,7 @@
 
 The RAM disk driver is a way to use main system memory as a block device.  It
 is required for initrd, an initial filesystem used if you need to load modules
-in order to access the root filesystem (see Documentation/initrd.txt).  It can
+in order to access the root filesystem (see Documentation/admin-guide/initrd.rst).  It can
 also be used for a temporary filesystem for crypto work, since the contents
 are erased on reboot.