Use "[api]" as log prefix instead of "[http]"

The old prefix could be confusing if the homeserver URL used the https://
scheme, and the new one better conveys that it is matrix.api who is doing
the logging.

matrix/api.lua

@@ -10,7 +10,7 @@ local json = require "cjson"
 
 local function noprintf(...) end
 local function eprintf(fmt, ...)
-   io.stderr:write("[http] ")
+   io.stderr:write("[api] ")
    io.stderr:write(fmt:format(...))
    io.stderr:write("\n")
    io.stderr:flush()