Small grammar correction

Here's explanation why this is the correct way:
https://english.stackexchange.com/questions/222316/are-you-sure-to-delete-or-are-you-sure-you-want-to-delete
Also `Are you sure you want to` could be found in 4 other messages in this same file.

Close #5848

PowerEditor/installer/nativeLang/english.xml

@@ -1012,7 +1012,7 @@ Continue?"/>
             <DeleteFileFailed title="Delete File" message="Delete File failed"/>
 
             <NbFileToOpenImportantWarning title="Amount of files to open is too large" message="$INT_REPLACE$ files are about to be opened.
-Are you sure to open them?"/>
+Are you sure you want to open them?"/>
             <SettingsOnCloudError title="Settings on Cloud" message="It seems the path of settings on cloud is set on a read only drive,
 or on a folder needed privilege right for writing access.
 Your settings on cloud will be canceled. Please reset a coherent value via Preference dialog."/>