Spelling fixes of a header for Asciidoc generation (#15988)

The case sensitivity for the heading for the response codes
was incorrect and has been corrected for the generation of Asciidoc
files.

Author: Oliver B. Fischer

modules/openapi-generator/src/main/resources/asciidoc-documentation/index.mustache

@@ -96,7 +96,7 @@ Operation Id:: {{nickname}}
 
 ===== Responses
 
-.http response codes
+.HTTP Response Codes
 [cols="2,3,1"]
 |===
 | Code | Message | Datatype