Improve var name consistency in BindingBenchmark

src/benchmark/kotlin/com/github/michaelbull/result/BindingBenchmark.kt

@@ -39,10 +39,9 @@ class BindingBenchmark {
 
     @Benchmark
     fun andThenSuccess(blackhole: Blackhole) {
-        val result =
-            provideX().andThen { first ->
-                provideY().andThen { second ->
-                    Ok(first + second)
+        val result = provideX().andThen { x ->
+            provideY().andThen { y ->
+                Ok(x + y)
             }
         }
 
@@ -51,10 +50,9 @@ class BindingBenchmark {
 
     @Benchmark
     fun andThenFailure(blackhole: Blackhole) {
-        val result =
-            provideX().andThen { first ->
-                provideZ().andThen { second ->
-                    Ok(first + second)
+        val result = provideX().andThen { x ->
+            provideZ().andThen { z ->
+                Ok(x + z)
             }
         }