[voice-tone] fix link

Fix the wrong fragment: 5th -> fifth in voice-tone.md.

Author: xiaoyinl

styleguide/voice-tone.md

@@ -6,7 +6,7 @@ help with that. Our style guide contains four recommendations:
 - [Use a Conversational Tone](#use-a-conversational-tone)
 - [Write in Second Person](#write-in-2nd-person)
 - [Use Active Voice](#use-active-voice)
-- [Target a 5th Grade Reading Level](#target-a-5th-grade-reading-level)
+- [Target a 5th Grade Reading Level](#target-a-fifth-grade-reading-level)
 
 You will see examples of each of these as you read this style guide. We've written this guide
 following our guidelines to provide examples for you to follow as you build documentation
@@ -75,4 +75,4 @@ reading your articles will have a formal computer science degree. Terms like 'id
 need to be defined if you use it:
 
 >The Close() method is idempotent, meaning that you can call it multiple times and the effect is
-the same as if you called it once.
+the same as if you called it once.