docs: added voice selection (#193)

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:
- [ ] Make sure to open an issue as a [bug/issue](https://github.com/googleapis/nodejs-dialogflow-cx/issues/new/choose) before writing your code!  That way we can discuss the change, evaluate designs, and agree on the general idea
- [ ] Ensure the tests and linter pass
- [ ] Code coverage does not decrease (if any source code was changed)
- [ ] Appropriate docs were updated (if necessary)

Fixes #<issue_number_goes_here> 🦕

Author: galz10

dialogflow-cx/detect-intent-streaming.js

@@ -93,6 +93,13 @@ async function main(
           config: {
             audioEncoding: encoding,
             sampleRateHertz: sampleRateHertz,
+            synthesize_speech_config: {
+              voice: {
+                // Set's the name and gender of the ssml voice
+                name: 'en-GB-Standard-A',
+                ssml_gender: 'SSML_VOICE_GENDER_FEMALE',
+              },
+            },
             singleUtterance: true,
           },
         },