docs(power): step-operations.md uses incorrect string literals for step semantics instead of StepSemantics enum

[TrickSumo]

issue

step-operations.md uses incorrect string literals for step semantics instead of StepSemantics enum

// Wrong
{ semantics: 'AT_LEAST_ONCE' }
{ semantics: 'AT_MOST_ONCE' }

Description of changes:

import { StepSemantics } from '@aws/durable-execution-sdk-js';

// Correct
{ semantics: StepSemantics.AtLeastOncePerRetry }
{ semantics: StepSemantics.AtMostOncePerRetry }