Fix spelling and grammar

Author: aldas

_fixture/certs/README.md

@@ -1,8 +1,13 @@
-To Generate valid certificate and private key use following command:
+To generate a valid certificate and private key use following command:
 
 ```bash
 # In OpenSSL ≥ 1.1.1
 openssl req -x509 -newkey rsa:4096 -sha256 -days 9999 -nodes \
   -keyout key.pem -out cert.pem -subj "/CN=localhost" \
   -addext "subjectAltName=DNS:localhost,IP:127.0.0.1,IP:::1"
 ```
+
+To check a certificate use following command:
+```bash
+openssl x509 -in cert.pem -text
+```