Fix code and typo in Fixpoints page (#67)

[chacham]

Fix #67

• Fix not working example code in "Using Flix to Solve Constraints on Lattices" section (#67)
  • Currently, MultiSelect for fixpoint select part uses , as seperator
    • https://github.com/flix/flix/blob/master/main/src/ca/uwaterloo/flix/language/phase/Parser.scala#L1093-L1095
  • Comparision method name at PartialOrder to lessEqual from partialCompare .
    • https://github.com/flix/flix/blob/master/main/src/library/PartialOrder.flix#L25
  • I don't know exactly why, (because I'm studying), instance Boxable[Sign] line is needed

It seems that the description should be modified as well by modifying the code. But since English is not my native language, it is a little difficult. Please help.

And there are some ignored whitespace which are reducing readability, so I added  s, and it looks ugly to me. ㅠ_ㅠ

[magnus-madsen]

Hello @chacham and welcome to our community!

Thanks for these edits. I have will have a look :)

[magnus-madsen]

Your changes look good, so I am merging them.

They will appear in the website the next time we build and deploy (could be a bit, since its a manual process).

I agree the documentation could use some more detail; we will work on that.