Fix a typo

Co-authored-by: Lukas Wirth <[email protected]>

src/syntax-extensions/expansion.md

@@ -107,7 +107,7 @@ as many as is needed to completely expand all invocations.
 Well, not *quite*.
 In fact, the compiler imposes an upper limit on the number of such recursive passes it is willing to run before giving up.
 This is known as the syntax extension recursion limit and defaults to 128.
-If the 128nd expansion contains a syntax extension invocation, the compiler will abort with an error indicating that the recursion limit was exceeded.
+If the 128th expansion contains a syntax extension invocation, the compiler will abort with an error indicating that the recursion limit was exceeded.
 
 This limit can be raised using the `#![recursion_limit="…"]` [attribute][recursion_limit], though it *must* be done crate-wide.
 Generally, it is recommended to try and keep syntax extension below this limit wherever possible as it may impact compilation times.