Update Localizable.strings

Fix reported on transifex
https://www.transifex.com/nextcloud/nextcloud/translate/#de_DE/ios-localizable/92030738?issue=yes

iOSClient/Supporting Files/en.lproj/Localizable.strings

@@ -39,7 +39,7 @@
 "_home_"                    = "Files";
 "_file_to_upload_"          = "File to upload";
 "_destination_"             = "Destination";
-"_ok_"                      = "Ok";
+"_ok_"                      = "OK";
 "_beta_version_"            = "Beta version";
 "_function_in_testing_"     = "Function in testing, please send information to any problems or malfunctions.";
 "_done_"                    = "Done";