Fix double quotation from full-width character (#107)

Author: jupemara

docs/README.md

@@ -78,7 +78,7 @@ curl -d "@mockPubsub.json" -X POST \
   -H "Ce-Specversion: true" \
   -H "Ce-Source: true" \
   -H "Ce-Id: true" \
-  -H “Content-Type: application/json” \
+  -H "Content-Type: application/json" \
   http://localhost:8080
 ```