Fix certificate file name

INSTALLATION.md

@@ -90,7 +90,7 @@ docker run -p 3000:3000 -v ~/data:/databunker/data \
 
 ```
 
-So, you need to prepare server.crt and server.key files.
+So, you need to prepare server.cer and server.key files.
 
 ## Generate self-signed certificates
 
@@ -102,14 +102,14 @@ mkdir -p certs
 cd certs
 openssl req -new -newkey rsa:4096 -days 365 -nodes -x509 \
     -subj "/C=UK/ST=/L=London/O=Your-company Ltd./CN=databunker.your-company.com" \
-    -keyout server.key -out server.crt
+    -keyout server.key -out server.cer
 ```
 
 ## Use certificates generated by letsencrypt
 
 Copy letsencrypt file **privkey.pem** to ~/certs/server.key
 
-Copy letsencrypt file **fullchain.pem** file to ~/certs/server.crt
+Copy letsencrypt file **fullchain.pem** file to ~/certs/server.cer
 
 
 # Create a test record