Fixes menu for rustdoc page (#804)

Author: FedericoPonzi

src/SUMMARY.md

@@ -64,7 +64,6 @@
 - [Prologue](./part-3-intro.md)
 - [Command-line arguments](./cli.md)
 - [The Rustc Driver and Interface](./rustc-driver.md)
-    - [Rustdoc](./rustdoc.md)
     - [Ex: Type checking through `rustc_interface`](./rustc-driver-interacting-with-the-ast.md)
     - [Ex: Getting diagnostics through `rustc_interface`](./rustc-driver-getting-diagnostics.md)
 - [Syntax and the AST](./syntax-intro.md)