bump supported version of protobuf to 3.4

docs/cn/getting_started.md

@@ -165,9 +165,9 @@ unittests can't be compiled with clang yet.
 
 no known issues.
 
-## protobuf: 2.4-3.2
+## protobuf: 2.4-3.4
 
-Be compatible with pb 3.0 and pb 2.x with the same file:
+Be compatible with pb 3.x and pb 2.x with the same file:
 Don't use new types in proto3 and start the proto file with `syntax="proto2";`
 [tools/add_syntax_equal_proto2_to_all.sh](https://github.com/brpc/brpc/blob/master/tools/add_syntax_equal_proto2_to_all.sh)can add `syntax="proto2"` to all proto files without it.
 protobuf 3.3-3.4 is not tested yet.