Fix build status badge

As mentioned in the GitHub Actions documentation [1]

> Note: Referencing the workflow file using the file path does not work
  if the workflow has a name.

[1] https://docs.github.com/en/free-pro-team@latest/actions/managing-workflow-runs/adding-a-workflow-status-badge

.github/workflows/tests.yml

@@ -1,4 +1,4 @@
-name: Test suite
+name: tests
 on:
   push:
     branches:
@@ -8,7 +8,7 @@ on:
       - i18n_master
 
 jobs:
-  test:
+  tests:
     runs-on: ubuntu-18.04
     services:
       postgres: