Clarify language

OpenKeychain/src/main/res/layout/certify_key_activity.xml

@@ -35,7 +35,7 @@
             android:layout_height="wrap_content"
             android:layout_marginBottom="4dp"
             android:layout_marginTop="14dp"
-            android:text="KEY TO SIGN" />
+            android:text="@string/section_key_to_certify" />
 
         <TableLayout
             android:layout_width="wrap_content"