use 'os' instead of 'platform' for job matrices

It's just a variable name, but the upstream docs now consistently use 'os'.

Fixes #8.

Author: ghostsquad

.github/workflows/test.yml

@@ -5,8 +5,8 @@ jobs:
     strategy:
       matrix:
         go-version: [1.13.x, 1.14.x]
-        platform: [ubuntu-latest, macos-latest, windows-latest]
-    runs-on: ${{ matrix.platform }}
+        os: [ubuntu-latest, macos-latest, windows-latest]
+    runs-on: ${{ matrix.os }}
     steps:
     - name: Install Go
       uses: actions/setup-go@v2

README.md

@@ -14,7 +14,7 @@ jobs:
     strategy:
       matrix:
         go-version: [1.13.x, 1.14.x]
-        platform: [ubuntu-latest, macos-latest, windows-latest]
+        os: [ubuntu-latest, macos-latest, windows-latest]
     runs-on: ${{ matrix.platform }}
     steps:
     - name: Install Go