Fixed a small typo in a Styling React Applications lesson

[WadieZN]

I fixed a small typo in "Styling React Applications lesson" in the first pararaph

from:

-- you'll have learned

into :

++ you have learned

[MaoShizhong]

"you'll have learned" (short for "you will have learned") is grammatically fine here, so no changes are required.
Closing as no action necessary.