docs generation: fix typo and remove trailing white space (#3972)

## What type of PR is this? (check all applicable)

- [ ] Refactor
- [ ] Feature
- [ ] Bug Fix
- [ ] Optimization
- [x] Documentation Update
- [ ] Community Node Submission


## Have you discussed this change with the InvokeAI team?
- [ ] Yes
- [x] No, because: This is a minor fix that I happened upon while
reading

      
## Have you updated all relevant documentation?
- [x] Yes
- [ ] No


## Description

Within the `mkdocs.yml` file, there's a typo where `Model Merging` is
spelled as `Model Mergeing`. I also found some unnecessary white space
that I removed.


## Related Tickets & Documents

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## QA Instructions, Screenshots, Recordings

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## Added/updated tests?

- [ ] Yes
- [x] No : Not big enough of a change to require tests (unless it is)

## [optional] Are there any post deployment tasks we need to perform?
Might need to re-run the yml file for docs to regenerate, but I'm hardly
familiar with the codebase so 🤷

mkdocs.yml

@@ -129,7 +129,7 @@ nav:
       - ControlNet: 'features/CONTROLNET.md'
       - Image-to-Image: 'features/IMG2IMG.md'
       - Controlling Logging: 'features/LOGGING.md'
-      - Model Mergeing: 'features/MODEL_MERGING.md'
+      - Model Merging: 'features/MODEL_MERGING.md'
       - Nodes Editor (Experimental): 'features/NODES.md'
       - NSFW Checker: 'features/NSFW.md'
       - Postprocessing: 'features/POSTPROCESS.md'
@@ -161,5 +161,3 @@ nav:
     - Other:
       - Contributors: 'other/CONTRIBUTORS.md'
       - CompViz-README: 'other/README-CompViz.md'
-
-