Rename past pset files so they sort correctly

bb109c2a · Erik Strand · 8a367a29 · bb109c2a · bb109c2a · bb109c2a
Commit bb109c2a authored 5 years ago by Erik Strand
--- a/_psets/1.md
+++ b/_psets/1.md
--- a/_psets/2.md
+++ b/_psets/2.md
--- a/_psets/3.md
+++ b/_psets/3.md
--- a/_psets/4.md
+++ b/_psets/4.md
--- a/_psets/5.md
+++ b/_psets/5.md
--- a/_psets/6.md
+++ b/_psets/6.md
--- a/_psets/7.md
+++ b/_psets/7.md
--- a/_psets/8.md
+++ b/_psets/8.md
--- a/_psets/9.md
+++ b/_psets/9.md