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

not obvious what Ex is, so rather get rid (#1372) · compiler-errors/rust@048d925

Original file line number	Diff line number	Diff line change
`@@ -76,8 +76,8 @@`
`76`	`76`	`- [Prologue](./part-3-intro.md)`
`77`	`77`	`- [Command-line arguments](./cli.md)`
`78`	`78`	`- [The Rustc Driver and Interface](./rustc-driver.md)`
`79`		-- [Ex: Type checking through `rustc_interface`](./rustc-driver-interacting-with-the-ast.md)
`80`		-- [Ex: Getting diagnostics through `rustc_interface`](./rustc-driver-getting-diagnostics.md)
	`79`	`+- [Example: Type checking](./rustc-driver-interacting-with-the-ast.md)`
	`80`	`+- [Example: Getting diagnostics](./rustc-driver-getting-diagnostics.md)`
`81`	`81`	`- [Syntax and the AST](./syntax-intro.md)`
`82`	`82`	`- [Lexing and Parsing](./the-parser.md)`
`83`	`83`	`- [Macro expansion](./macro-expansion.md)`