fix(AIP-4234): fix typo ListBooksRequest (#1159)

rpc request should match rpc name.

fixes #1150

Co-authored-by: Noah Dietz <noahdietz@users.noreply.github.com>

Author: toumorokoshi

aip/client-libraries/4234.md

@@ -105,7 +105,7 @@ example, take the following service definition:
 
 ```proto
 service LibraryService {
-  rpc ListBooks(ListBookRequest) returns (ListBooksResponse);
+  rpc ListBooks(ListBooksRequest) returns (ListBooksResponse);
 
   // Other host API RPCs...