Documentation: admin-guide: Update kvm/xen config option

Since commit 9bba03d4 ("kconfig: remove 'kvmconfig' and 'xenconfig'
shorthands") kvm/xen config shortcuts are not available anymore. Update
the file to reflect how they should be used, with the full filename.
Signed-off-by: André Almeida <andrealmeid@collabora.com>
Link: https://lore.kernel.org/r/20210130014547.123006-2-andrealmeid@collabora.comSigned-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/admin-guide/README.rst

@@ -226,10 +226,11 @@ Configuring the kernel
                            all module options to built in (=y) options. You can
                            also preserve modules by LMC_KEEP.
 
-     "make kvmconfig"   Enable additional options for kvm guest kernel support.
+     "make kvm_guest.config"   Enable additional options for kvm guest kernel
+                               support.
 
-     "make xenconfig"   Enable additional options for xen dom0 guest kernel
-                        support.
+     "make xen.config"   Enable additional options for xen dom0 guest kernel
+                         support.
 
      "make tinyconfig"  Configure the tiniest possible kernel.