Updated SinglyLinkedList.java, added a rotate Function

Describe your change:

• [x] --> Added A Function that rotates the LinkedList by K nodes where K is less than or equal to the length of LinkedList.

Checklist:

• [x] --> I have read CONTRIBUTING.md.
• [x] --> This pull request is all my own work -- I have not plagiarized.
• [x] --> I know that pull requests will not be merged if they fail the automated tests.
• [x] --> This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• [x] --> All functions and variable names follow Java naming conventions.

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