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>
diff --git a/doc/README.distro b/doc/README.distro
index 3715c8c..e1b7216 100644
--- a/doc/README.distro
+++ b/doc/README.distro
@@ -243,7 +243,7 @@
 
   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.