doc: don't mention -memory, it is -m

Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Markus Armbruster <armbru@redhat.com>

qemu-options.hx

@@ -250,7 +250,7 @@ use is discouraged as it may be removed from future versions.
 ETEXI
 
 DEF("m", HAS_ARG, QEMU_OPTION_m,
-    "-m[emory] [size=]megs[,slots=n,maxmem=size]\n"
+    "-m [size=]megs[,slots=n,maxmem=size]\n"
     "                configure guest RAM\n"
     "                size: initial amount of guest memory\n"
     "                slots: number of hotplug slots (default: none)\n"