chore(doc): Fix proto code block (#778)

Co-authored-by: Lucio Franco <luciofranco14@gmail.com>

README.md

@@ -73,7 +73,7 @@ a Rust module. For example, given the `package` specifier:
 
 [package]: https://developers.google.com/protocol-buffers/docs/proto#packages
 
-```proto,ignore
+```protobuf,ignore
 package foo.bar;
 ```
 
@@ -83,7 +83,7 @@ All Rust types generated from the file will be in the `foo::bar` module.
 
 Given a simple message declaration:
 
-```proto,ignore
+```protobuf,ignore
 // Sample message.
 message Foo {
 }
@@ -131,7 +131,7 @@ All `.proto` enumeration types convert to the Rust `i32` type. Additionally,
 each enumeration type gets a corresponding Rust `enum` type. For example, this
 `proto` enum:
 
-```proto,ignore
+```protobuf,ignore
 enum PhoneType {
   MOBILE = 0;
   HOME = 1;
@@ -186,7 +186,7 @@ message will have 'accessor' methods to get/set the value of the field as the
 Rust enum type. For instance, this proto `PhoneNumber` message that has a field
 named `type` of type `PhoneType`:
 
-```proto,ignore
+```protobuf,ignore
 message PhoneNumber {
   string number = 1;
   PhoneType type = 2;
@@ -257,7 +257,7 @@ Oneof fields convert to a Rust enum. Protobuf `oneof`s types are not named, so
 defines the enum in a module under the struct. For example, a `proto3` message
 such as:
 
-```proto,ignore
+```protobuf,ignore
 message Foo {
   oneof widget {
     int32 quux = 1;
@@ -294,7 +294,7 @@ application's specific needs.
 
 Example `.proto` file:
 
-```proto,ignore
+```protobuf,ignore
 syntax = "proto3";
 package tutorial;