Added single quotes around format value (home-assistant#4609)

Marko Korhonen · fabaff · commit de28af78b600 · 2018-02-09T20:40:55.000+01:00
The service wont work without these
diff --git a/source/_components/tts.voicerss.markdown b/source/_components/tts.voicerss.markdown
@@ -41,7 +41,7 @@ tts:
     api_key: 'XXXXX'
     language: 'de-de'
     codec: mp3
-    format: 8khz_8bit_mono
+    format: '8khz_8bit_mono'
 ```
 
 Please note, some media_players require a certain format. For example the Sonos requires a format of '44khz_16bit_stereo'
