Rename class to match filename

The name of this public class should match the filename.

Fixes #3493

COPYBARA_INTEGRATE_REVIEW=#3493 from glenacota:patch-1 76a8c4f
PiperOrigin-RevId: 499143657

core/src/test/java/com/google/errorprone/refaster/testdata/template/UnnecessaryLambdaParens.java

@@ -22,7 +22,7 @@
 import java.util.stream.Collectors;
 import java.util.stream.Stream;
 
-public class UnnecessaryLambdaParen<T> {
+public class UnnecessaryLambdaParens<T> {
   @BeforeTemplate
   Optional<T> last(Stream<T> stream) {
     return stream.map((x) -> x).collect(Collectors.reducing((a, b) -> b));