push CPU to max 16, show how to edit CPU and memory at once

In team#41597, i messed up because I did `-B vcpus=4 -B
memory=8g` and then only the memory was changed!

howto/ganeti.md

@@ -323,8 +323,7 @@ half-open filesystems and stray DRBD devices, do take a look at the
 It's possible to change the IP, CPU, or memory allocation of an instance
 using the [gnt-instance modify](http://docs.ganeti.org/docs/ganeti/3.0/html/man-gnt-instance.html#modify) command:
 
-    gnt-instance modify -B vcpus=4 test1.torproject.org
-    gnt-instance modify -B memory=8g test1.torproject.org
+    gnt-instance modify -B vcpus=4,memory=8g test1.torproject.org
     gnt-instance reboot test1.torproject.org
 
 ### IP address change
@@ -2805,7 +2804,7 @@ Cluster limits were changed to raise the disk usage to 2TiB:
 
 ```
 gnt-cluster modify --ipolicy-bounds-specs \
-max:cpu-count=8,disk-count=16,disk-size=2097152,\
+max:cpu-count=16,disk-count=16,disk-size=2097152,\
 memory-size=32768,nic-count=8,spindle-use=12\
 /min:cpu-count=1,disk-count=1,disk-size=1024,\
 memory-size=128,nic-count=1,spindle-use=1