doc: fix default value for maxHeadersCount in http.md

The documentation previously stated that the default for
`server.maxHeadersCount` and `request.maxHeadersCount` was 2000.
However, `MAX_HEADER_PAIRS` in `lib/_http_common.js` limits parser
pairs (header name and value strings) to 2000, which corresponds to
an effective default limit of 1000 headers.

Fixes: #65470
Signed-off-by: Santusht kotai <115890693+santusht06@users.noreply.github.com>

Author: santusht06