Ohhh the should_panic was for mdbook test. Oops

`panics` is the ferris class, which was why ferris didn't show up on
these. The doc tests don't pass CLI args either, so these will panic
during tests, and we don't want to ignore them because we do want to
compile them.

This should fix the build 🤞

src/ch12-01-accepting-command-line-arguments.md

@@ -102,7 +102,7 @@ throughout the rest of the program. We do that in Listing 12-2.
 
 <span class="filename">Filename: src/main.rs</span>
 
-```rust,noplayground
+```rust,should_panic,noplayground
 {{#rustdoc_include ../listings/ch12-an-io-project/listing-12-02/src/main.rs}}
 ```
 

src/ch12-02-reading-a-file.md

@@ -22,7 +22,7 @@ shown in Listing 12-4.
 
 <span class="filename">Filename: src/main.rs</span>
 
-```rust,noplayground
+```rust,should_panic,noplayground
 {{#rustdoc_include ../listings/ch12-an-io-project/listing-12-04/src/main.rs:here}}
 ```
 

src/ch12-03-improving-error-handling-and-modularity.md

@@ -119,7 +119,7 @@ Listing 12-6 shows the improvements to the `parse_config` function.
 
 <span class="filename">Filename: src/main.rs</span>
 
-```rust,noplayground
+```rust,should_panic,noplayground
 {{#rustdoc_include ../listings/ch12-an-io-project/listing-12-06/src/main.rs:here}}
 ```
 
@@ -187,7 +187,7 @@ shows the changes we need to make.
 
 <span class="filename">Filename: src/main.rs</span>
 
-```rust,noplayground
+```rust,should_panic,noplayground
 {{#rustdoc_include ../listings/ch12-an-io-project/listing-12-07/src/main.rs:here}}
 ```