build: Update lean and add some gitignores

.gitignore

@@ -1,2 +1,3 @@
 /.lake
 .directory
+qr-code.png

Lean Talk SP24/basics.lean

@@ -13,6 +13,8 @@ open Nat
 def f (x : ℕ) :=
   x + 3
 
+#eval f 5
+
 #check f
 
 -- These are propositions, of type `Prop`.

lake-manifest.json

@@ -58,7 +58,7 @@
   {"url": "https://github.com/leanprover-community/mathlib4.git",
    "type": "git",
    "subDir": null,
-   "rev": "328b7ecbc986507635abf725bbed4da70e418298",
+   "rev": "e415c3ba4d1e59e5af6d0b9b4f71797f97cad62a",
    "name": "mathlib",
    "manifestFile": "lake-manifest.json",
    "inputRev": null,