pw_digital_io: fix typo in docs

Replace reference to nonexitent class `DigitalIoIn` with `DigitalIn`.

Change-Id: I12c58efe3f43a663e6439bca8a535d219bd005f2
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/99422
Commit-Queue: Anton Markov <amarkov@google.com>
Reviewed-by: Wyatt Hepler <hepler@google.com>

pw_digital_io/docs.rst

@@ -248,7 +248,7 @@ Backend Implemention Notes
   declare ``GetState`` public and ``SetState`` private.
 * Derived classes that exclude a particular functionality provide a private,
   final implementation of the unsupported virtual method that crashes if it is
-  called. For example, ``DigitalIoIn`` implements ``DoSetState`` to trigger
+  called. For example, ``DigitalIn`` implements ``DoSetState`` to trigger
   ``PW_CRASH``.
 * Backend implementations provide real implementation for the remaining pure
   virtual functions of the class they extend.