Update release_6.3.htm

[rmapree]

Editing release_6.3.htm to include changes made to scatter estimator so that the tail fitting scale factors are correct.

Changes in this pull request

Updating documentation for release_6.3.htm

Checklist before requesting a review

• I have performed a self-review of my code
• I have added docstrings/doxygen in line with the guidance in the developer guide
• [] I have implemented unit tests that cover any new or modified functionality (if applicable)
• The code builds and runs on my machine
• documentation/release_XXX.md has been updated with any functionality change (if applicable)