Update 7.2.md
Update 7.2.md

Adds more detail for 7.2-4 and 7.2-5
gzc/CLRS,Soyn/CLRS,semsevens/CLRS,gzc/CLRS,Soyn/CLRS,gzc/CLRS,semsevens/CLRS,semsevens/CLRS,Soyn/CLRS,Jingru/CLRS,Jingru/CLRS,Jingru/CLRS