Fix Bug 440041 - Sieve editor does not show the line numbers correctly

BUG: 440041
FIXED-IN: 5.18.0

src/ksieveui/editor/sievetextedit.cpp

@@ -107,7 +107,7 @@ int SieveTextEdit::lineNumberAreaWidth() const
         ++digits;
     }
 
-    const int space = 2 + fontMetrics().boundingRect(QLatin1Char('9')).width() * digits;
+    const int space = 2 + fontMetrics().boundingRect(QLatin1Char('X')).width() * digits;
     return space;
 }