not obvious what Ex is, so rather get rid (rust-lang#1372)

Author: tshepang

src/SUMMARY.md

@@ -76,8 +76,8 @@
 - [Prologue](./part-3-intro.md)
 - [Command-line arguments](./cli.md)
 - [The Rustc Driver and Interface](./rustc-driver.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)
+    - [Example: Type checking](./rustc-driver-interacting-with-the-ast.md)
+    - [Example: Getting diagnostics](./rustc-driver-getting-diagnostics.md)
 - [Syntax and the AST](./syntax-intro.md)
     - [Lexing and Parsing](./the-parser.md)
     - [Macro expansion](./macro-expansion.md)