Update LMM fitting subheadings on Github page

[s2hui]

Thanks for making this very useful package! I was reading the example information on the main page of the github and under the section "LMM fitting", there are 3 options that users can use depending on the size of their data set. The options are numbered 1, 2, 3 however it can be mistakenly interpreted by beginners as Step 1, Step 2, Step 3. Just a suggestion to retitle these options as "Option 1", "Option 2" and "Option 3" to make things very clear for new users.

[ChangjiangXu]

Thanks for the comments. A change was made.