Fix string template in rebaseline_tests.py. NFC (emscripten-core#27354)

Author: sbc100

tools/maint/rebaseline_tests.py

@@ -203,7 +203,7 @@ def main():
   if args.check_only:
     message = 'Test expectations are out-of-date\n'
   elif args.update_emsdk:
-    message = f'''Update emsdk version from ${current_sha} => ${installed_sha}
+    message = f'''Update emsdk version from {current_sha} to {installed_sha}
 
 This is an automatic change generated by tools/maint/rebaseline_tests.py --update-emsdk.