doc - use ${variable} for string place holder

[Bang Pham Huu]

Rasdaman doc should use one format for variable. The selected format is ${variable}, e.g:

http(s)://<endpoint url>

should be

http(s)://{endpoint-url}  (default: http://localhost:8080/rasdaman/ows)

[Dimitar Misev]

Just use ​http://localhost:8080/rasdaman/ows instead of http(s):${endpoint url}

[Peter Baumann]

let us use abbreviations indeed, and the URL already contains the URL scheme so we do not need "http(s):".