Fix example usage of headers in configuration file (lycheeverse#1540)

lychee.example.toml

@@ -69,7 +69,7 @@ require_https = false
 method = "get"
 
 # Custom request headers
-headers = []
+header = ["name=value", "other=value"]
 
 # Remap URI matching pattern to different URI.
 remap = ["https://example.com http://example.invalid"]