balloon doc fix

diff -r a83ac0806d6b docs/src/user.tex
@@ -956,11 +956,11 @@
 
 \subsection{Setting memory footprints from within a domain}
 
-The virtual file \path{/proc/xen/memory\_target} allows the owner of a
+The virtual file \path{/proc/xen/balloon} allows the owner of a
 domain to adjust their own memory footprint.  Reading the file
-(e.g. \path{cat /proc/xen/memory\_target}) prints out the current
+(e.g. \path{cat /proc/xen/balloon}) prints out the current
 memory footprint of the domain.  Writing the file
-(e.g. \path{echo new\_target > /proc/xen/memory\_target}) requests
+(e.g. \path{echo new\_target > /proc/xen/balloon}) requests
 that the kernel adjust the domain's memory footprint to a new value.
 
 \subsection{Setting memory limits}