Fix obsolete Web Chat API of Avatar

[nt-7]

Description

I found botAvatarInitials and userAvatarInitials on README document.
However, it doesn't work.

Also, I found there is no longer the two parameters on Web Chat API Reference.

Meanwhile, it works correctly if I set botAvatarInitials and userAvatarInitials on styleOptions.

Specific Changes

I remove botAvatarInitials and userAvatarInitials from Web Chat API parameters on README document.

before

window.WebChat.renderWebChat(
        {
          directLine: window.WebChat.createDirectLine({
            token: 'YOUR_DIRECT_LINE_TOKEN'
          }),
          userID: 'YOUR_USER_ID',
          username: 'Web Chat User',
          locale: 'en-US',
          botAvatarInitials: 'WC',
          userAvatarInitials: 'WW'
        },
        document.getElementById('webchat')
      );

after

window.WebChat.renderWebChat(
        {
          directLine: window.WebChat.createDirectLine({
            token: 'YOUR_DIRECT_LINE_TOKEN'
          }),
          userID: 'YOUR_USER_ID',
          username: 'Web Chat User',
          locale: 'en-US'
        },
        document.getElementById('webchat')
      );

Previously it could be set in the Web Chat API Reference, but I'm guessing it's obsolete now.
I believe it is appropriate to set them in styleOptions now.

[corinagum]

LGTM, thank you!