docs: update protoc version from 3.13.0 to 3.15.5 (grpc#709)

Signed-off-by: Eng Zer Jun <[email protected]>

Author: Juneezee

content/en/docs/protoc-installation.md

@@ -2,7 +2,7 @@
 title: Protocol Buffer Compiler Installation
 short_title: Protoc Installation
 description: How to install the protocol buffer compiler.
-protoc-version: 3.13.0
+protoc-version: 3.15.5
 toc_hide: true
 ---