Update SinglyLinkedList.java

[dave-sh]

added sort LinkedList

Describe your change:

Added sort linked list

References

<Sort a Linked List #2450>

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Java files are placed inside an existing directory.
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• If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

[siriak]

There's an implementation of sorting linked list here https://github.com/TheAlgorithms/Java/blob/857c4aafb27f2d1a3db41dd393c90e07a9d656b4/src/main/java/com/thealgorithms/sorts/LinkList_Sort.java. Could you update it with your improvements?

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