Fix rust-by-example spanish translation

A spanish translation was added in
rust-lang/rust-by-example#1910, but the upstream
integration was never added.

Author: ehuss

src/bootstrap/src/core/build_steps/doc.rs

@@ -73,7 +73,7 @@ book!(
     EditionGuide, "src/doc/edition-guide", "edition-guide", &[];
     EmbeddedBook, "src/doc/embedded-book", "embedded-book", &[];
     Nomicon, "src/doc/nomicon", "nomicon", &[];
-    RustByExample, "src/doc/rust-by-example", "rust-by-example", &["ja", "zh"];
+    RustByExample, "src/doc/rust-by-example", "rust-by-example", &["es", "ja", "zh"];
     RustdocBook, "src/doc/rustdoc", "rustdoc", &[];
     StyleGuide, "src/doc/style-guide", "style-guide", &[];
 );