TRPL: Add rust Marker to Some Code Block

This adds strictly more information to the source files and reduces the
need for customized tooling to render the book.

(While this should not change the output of _rustbook_, it is very
useful when rendering the sources with external tools like Pandoc.)

src/doc/trpl/method-syntax.md

@@ -89,7 +89,7 @@ So, now we know how to call a method, such as `foo.bar()`. But what about our
 original example, `foo.bar().baz()`? This is called ‘method chaining’, and we
 can do it by returning `self`.
 
-```
+```rust
 struct Circle {
     x: f64,
     y: f64,
@@ -117,7 +117,7 @@ fn main() {
 
 Check the return type:
 
-```
+```rust
 # struct Circle;
 # impl Circle {
 fn grow(&self) -> Circle {
@@ -167,7 +167,7 @@ and `y` attributes will be `0.0`, and the `radius` will be `1.0`. Rust doesn’t
 have method overloading, named arguments, or variable arguments. We employ
 the builder pattern instead. It looks like this:
 
-```
+```rust
 struct Circle {
     x: f64,
     y: f64,