Fixed documentation rendering.
See
//! 2. Non-terminals
//!
//! | Non-terminal | Usage |
-//! |--------------|------------------------------------------------------------|
+//! |-----------------------|------------------------------------------------------------|
//! | `(e)` | matches `e` |
//! | `e1 ~ e2` | matches the sequence `e1` `e2` |
-//! | `e1 | e2` | matches either `e1` or `e2` |
+//! | <code>e1 \| e2</code> | matches either `e1` or `e2` |
//! | `e*` | matches `e` zero or more times |
//! | `e+` | matches `e` one or more times |
//! | `e{n}` | matches `e` exactly `n` times |
