Match the right Ubuntu version to Erlang/OTP-20 in readme (#212)

README.md

@@ -121,7 +121,7 @@ on: push
 
 jobs:
   test:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     name: OTP ${{matrix.otp}} / Elixir ${{matrix.elixir}}
     strategy:
       matrix:
@@ -145,7 +145,7 @@ on: push
 
 jobs:
   test:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-20.04
     name: Erlang/OTP ${{matrix.otp}} / rebar3 ${{matrix.rebar3}}
     strategy:
       matrix: