Rename Reference page title (#1570)

## Describe your changes

Rename Reference page title.

## Checklist before requesting a review
- [ ] Add unit tests for this change.
- [ ] Make sure all tests can pass.
- [ ] Update documents if necessary.
- [ ] Lint and apply fixes to your code by running `lintrunner -a`
- [ ] Is this a user-facing change? If yes, give a description of this
change to be included in the release notes.
- [ ] Is this PR including examples changes? If yes, please remember to
update [example
documentation](https://github.com/microsoft/Olive/blob/main/docs/source/examples.md)
in a follow-up PR.

## (Optional) Issue link

Author: xiaoyu-work

docs/source/reference/index.rst

@@ -1,4 +1,4 @@
-API reference
+Reference
 ==============
 
 .. grid:: 2 2 2 3