Fix minor typos and file name in contributing docs (#1651)

# Description

*Please include a summary of the changes and the related issue. Please
also include relevant motivation and context,
including:*

- What does this PR do?

This PR fixes two minor typos in the contributing top level docs, and
fixes a broken link to another file.

- Why are these changes needed?

While reading the guidelines I noticed two small typos and a broken link
to the documentation guidelines in the same directory.

- How were these changes implemented and what do they affect?

Both typos were fixed. The file has been renamed from
`DOCUMENTATION_GUIDELINE.md` to `DOCUMENTATION_GUIDELINES.md` to match
the link, as the link seems correct vs the filename. There are no other
references to this file within this repo.

There are no open issues related to this

docs/CONTRIBUTING.md

@@ -43,7 +43,7 @@ The set of labels and their description can be found [here](https://paritytech.g
 2. Please tag each PR with minimum one `T*` label. The respective `T*` labels should signal the
    component that was changed, they are also used by downstream users to track changes and to
    include these changes properly into their own releases.
-3. If your’re still working on your PR, please submit as “Draft”. Once a PR is ready for review change
+3. If you’re still working on your PR, please submit as “Draft”. Once a PR is ready for review change
    the status to “Open”, so that the maintainers get to review your PR. Generally PRs should sit for
    48 hours in order to garner feedback. It may be merged before if all relevant parties had a look at it.
 4. If you’re introducing a major change, that might impact the documentation please add the label
@@ -143,4 +143,4 @@ UI tests are used for macros to ensure that the output of a macro doesn’t chan
 These UI tests are sensible to any changes in the macro generated code or to switching the rust stable version.
 The tests are only run when the `RUN_UI_TESTS` environment variable is set. So, when the CI is for example complaining
 about failing UI tests and it is expected that they fail these tests need to be executed locally.
-To simplify the updating of the UI test ouput there is the `.maintain/update-rust-stable
+To simplify the updating of the UI test output there is the `.maintain/update-rust-stable