Tweaks to step functions docs and tests (#1052)

Author: GrahamCampbell

tests/Integration/StepFunctionsClientTest.php

@@ -21,7 +21,7 @@ public function testStartExecution(): void
             'input' => 'change me',
             'traceHeader' => 'change me',
         ]);
-        $result = $client->StartExecution($input);
+        $result = $client->startExecution($input);
 
         $result->resolve();
 

tests/Unit/StepFunctionsClientTest.php

@@ -20,7 +20,7 @@ public function testStartExecution(): void
             'name' => 'qwertyuiop',
             'input' => '{"foo": "bar"}',
         ]);
-        $result = $client->StartExecution($input);
+        $result = $client->startExecution($input);
 
         self::assertInstanceOf(StartExecutionOutput::class, $result);
         self::assertFalse($result->info()['resolved']);