Fix variable documented in README.distro for PXE address.

Fixes typo of pxe_addr_r with pxefile_addr_r.
Signed-off-by: Vagrant Cascadian <vagrant@debian.org>
Reviewed-by: Stephen Warren <swarren@wwwdotorg.org>

doc/README.distro

@@ -243,7 +243,7 @@ kernel_addr_r:
 
   A size of 16MB for the kernel is likely adequate.
 
-pxe_addr_r:
+pxefile_addr_r:
 
   Mandatory. The location in RAM where extlinux.conf will be loaded to prior
   to processing.