Fix CI workflow YAML syntax

Author: inaciovasquez2020

.github/workflows/build.yml

@@ -1,21 +1,25 @@
 name: Lean build
 
-on: [push, pull_request]
+on:
+push:
+pull_request:
 
 jobs:
 build:
 runs-on: ubuntu-latest
-steps:
-- uses: actions/checkout@v4
 
 ```
+steps:
+  - name: Checkout repository
+    uses: actions/checkout@v4
+
   - name: Install Lean
     uses: leanprover/lean-action@v1
 
-  - name: Fetch cache
+  - name: Fetch mathlib cache
     run: lake exe cache get
 
-  - name: Build
+  - name: Build project
     run: lake build
 ```