I filed taxes for 2017 a month ago but I received a tax document from Robinhood recently. I gained only $5.31 by selling one share of stock. Should I do amend?

Although the amount of capital gain is small, it is always advisable to amend your return in this case. The IRS has a copy of your 1099-B and if they cannot find it on your return will issue a letter to tax you on the whole proceeds and not only the capital gains. You would then have to write an explanatory letter which will be more cumbersome than filing an amended return.

https://ttlc.intuit.com/replies/3288565

Your stock sale was almost certainly a covered transaction. IRS knows the basis and it is not zero.
The tax on a $5.00 gain is at most $1.00
IRS is not going to go after you for one dollar. It costs them more than a dollar to do that.