Removed extra | in documentation

[satya-vinay]

Why are these changes needed?

To make the Documentation cleaner.
The Tune-User-Defined-Function doc(i.e https://microsoft.github.io/FLAML/docs/Use-Cases/Tune-User-Defined-Function#details-and-guidelines-on-hyperparameter-search-space) had an extra | after the table.
So I had removed the extra |.

Related issue number

Closes #775

Screenshots

| Before |

| After |

Checks

• I've used pre-commit to lint the changes in this PR, or I've made sure lint with flake8 output is two 0s.
• I've included any doc changes needed for https://microsoft.github.io/FLAML/. See https://microsoft.github.io/FLAML/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[sonichi]

Could you attach a snapshot of the updated webpage? Thanks.
@skzhang1 please do not merge before confirming that.

[satya-vinay]

@sonichi @skzhang1, I have updated the main comment with screenshots. Please have a look

[skzhang1]

@sonichi @skzhang1, I have updated the main comment with screenshots. Please have a look

Looks good to me! Thanks.