Replace {@link ThreadSafeTypeParameter} with `{@code ThreadSafeType…

…Parameter}`

The external build puts type annotations in a separate artifact, this fixes:

```
Error:  /home/runner/work/error-prone/error-prone/annotations/src/main/java/com/google/errorprone/annotations/ThreadSafe.java:89: error: reference not found
Error:   * {@link ThreadSafeTypeParameter} javadoc.
Error:            ^
```
PiperOrigin-RevId: 668526430

annotations/src/main/java/com/google/errorprone/annotations/ThreadSafe.java

@@ -86,7 +86,7 @@
  * <p>This first requirement means the type is at least inherently shallowly thread-safe. For types
  * with type parameters to be deemed deeply thread-safe, those of these types that denote
  * containment must also be deemed thread-safe. A full explanation of this can be found in the
- * {@link ThreadSafeTypeParameter} javadoc.
+ * {@code ThreadSafeTypeParameter} javadoc.
  *
  * <p>Fields annotated with {@code javax.annotation.concurrent.GuardedBy} are likely the meat of a
  * mutable thread-safe class: these are things that need to be mutated, but should be done so in a