fix #750: punctuation

fix #750: punctuation

Following strings are concatened:
```
<string name="please_sign_in">Sign in again.</string>
<string name="incorrect_credentials">Incorrect username or password.</string>
```

Used in another package in a single toast message:
```
<string name="passcode_wrong_passcode">Wrong PIN</string>
```

Not sure what is the best for `reply_failed` and `tap_retry` used in NoteCommentFragment