Update strings.xml file

Delete unnecessary dot.

src/main/res/values/strings.xml

@@ -282,7 +282,7 @@
     <string name="create_dir_fail_msg">Could not create folder</string>
     <string name="filename_forbidden_characters">Forbidden characters: / \\ &lt; &gt; : " | ? *</string>
     <string name="filename_forbidden_charaters_from_server">Filename contains at least one invalid character</string>
-    <string name="filename_empty">Filename cannot be empty.</string>
+    <string name="filename_empty">Filename cannot be empty</string>
     <string name="wait_a_moment">Wait a moment…</string>
     <string name="wait_checking_credentials">Checking stored credentials</string>
     <string name="filedisplay_no_file_selected">No file selected</string>