[Flutter] TOTP Documentation issues/improvements

**Page:**
https://docs.amplify.aws/lib/auth/signin_next_steps/q/platform/flutter/#continue-signin-with-totp-setup

**Sentence:**
> For more advanced use cases, TOTPSetupDetails also contains a sharedSecret which can be used to either generate a QR code or be manually entered into an authenticator app.

**Issue:**
The sentence above make it sound like the QR code can be generated out of `sharedSecret`. But the URI generation that `getSetupUri` does is the one that would be working.

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**Page:**
https://docs.amplify.aws/lib/auth/signin_next_steps/q/platform/flutter/#continue-signin-with-totp-setup

**Sentence:**
`final setupUri = totpSetupDetails.getSetupUri(appName: 'MyApp');`

**Issue:**
The **MyApp** does not sound like the current app. It may also give a feeling to write the Authenticator app of user's choice.
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**Page:**
https://docs.amplify.aws/lib/auth/mfa/q/platform/flutter/#enabling-totp-after-a-user-is-signed-in

**Sentence:**
that contains the sharedSecret which will be used to either to generate a QR code or can be manually entered into an Authenticator app.

**Issue:**
The sentence starts with a lower case and feels random.