readme: update shield syntax (badges/shields#8671)

Signed-off-by: Unai Martinez-Corral <umartinezcorral@antmicro.com>

README.md

diff --git a/README.md b/README.md
index 5b66331..2940938 100644
--- a/README.md
+++ b/README.md
@@ -14,7 +14,7 @@
   -->
   <a title="Documentation Status" href="https://f4pga.readthedocs.io/projects/arch-defs/"><img alt="Documentation Status" src="https://img.shields.io/readthedocs/symbiflow-arch-defs/latest?longCache=true&style=flat-square&logo=ReadTheDocs&logoColor=fff&label=Architecture%20Definitions%20Documentation"></a><!--
   -->
-  <a title="'Automerge' workflow status" href="https://github.com/SymbiFlow/f4pga-arch-defs/actions/workflows/Automerge.yml"><img alt="'Automerge' workflow status" src="https://img.shields.io/github/workflow/status/SymbiFlow/f4pga-arch-defs/Automerge/main?longCache=true&style=flat-square&label=Tests&logo=Github%20Actions&logoColor=fff"></a><!--
+  <a title="'Automerge' workflow status" href="https://github.com/SymbiFlow/f4pga-arch-defs/actions/workflows/Automerge.yml"><img alt="'Automerge' workflow status" src="https://img.shields.io/github/actions/workflow/status/SymbiFlow/f4pga-arch-defs/Automerge.yml?branch=main&longCache=true&style=flat-square&label=Tests&logo=Github%20Actions&logoColor=fff"></a><!--
   -->
 </p>