Change the ui of the category dropdown in Transaction form

[ani1609]

Currently the ui of the category dropdown is

Change the ui to

The styling should be same but the dropdown contents should be same as of now)
(The form is present in client/src/components/TransactionForm.js component)

[SHIVAA17]

May I work on this issue

[ani1609]

@SHIVAA17 assigning

[saksham-jain177]

@ani1609 I would like to work on this