prost-build lib.rs: Minor doc comment edits (#498)

Fix the indentation of the example .proto file. Fix build.rs compile error.
Fixes:

error[E0107]: wrong number of type arguments: expected 2, found 1

Co-authored-by: Evan Jones <ej@evanjones.ca>

prost-build/src/lib.rs

@@ -36,14 +36,14 @@
 //!
 //! // A snazzy new shirt!
 //! message Shirt {
-//! enum Size {
-//!     SMALL = 0;
-//!     MEDIUM = 1;
-//!     LARGE = 2;
-//! }
+//!     enum Size {
+//!         SMALL = 0;
+//!         MEDIUM = 1;
+//!         LARGE = 2;
+//!     }
 //!
-//! string color = 1;
-//! Size size = 2;
+//!     string color = 1;
+//!     Size size = 2;
 //! }
 //! ```
 //!