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.
I think we want the warning to indicate the maximum number of files that can be replaced before it becomes undoable not just say "more than one". 

You could have 5 files that get searched and this message wouldn't appear even though it's more than one.