README: use killall instead of pkill

As discussed in the mailing list[1], pkill seems not
to be working in a very typical Linux setup.

[1] http://sourceforge.net/p/libmtp/mailman/libmtp-discuss/?viewmonth=201504&style=threaded&viewday=6
diff --git a/README b/README
index b9ac1e6..9d382a2 100644
--- a/README
+++ b/README
@@ -177,7 +177,7 @@
   Sometimes the "gvfs-gphoto2-volume-monitor" is running on the
   system and hogging the device, try something like:
 
-  pkill gvfs-gphoto2-volume-monitor
+  killall gvfs-gphoto2-volume-monitor
 
   Then plug in the device and issue "mtp-detect" to figure out if
   this may be the case.