fix help text in "Create An image" window

In the "Create An image" window, there are "minimum disk" and "minimun
memory" text input. But both help text on the two text inputs are using
"the minimum disk size...". Actually for the second one we should say
"the minimum memory size...", and now this patch fix this.

Change-Id: Ife19acba5fafb41d99e9416d2f9e8082b9eaef53
Merge "fix help text in "Create An image" window"