Update strings.js

This is a more clear explaination confirmed by a native speaker.
For "REPLACE_WITHOUT_UNDO_WARNING", if the {0} placeholder is used, then the grammar is too difficult.
Example:  
Since more than (one) file NEEDS to be modified.
Since more than (two) file NEED to be modified.

To resolve this I used Since more than one file needs to be modified, since it satifies 1 up to n files.
The replacement text {0} is the number of files that can be changed in memory before changes start to be made directly to files on disk which cannot be undone. By default this value is 20 but could change at any time.