Fix typo: Replace 'two' with 'three' in /docs/mcp.md (#757)

The documentation in `docs/mcp.md` listed three server types (stdio,
HTTP over SSE, Streamable HTTP) but incorrectly stated "two kinds of
servers" in the heading. This PR fixes the numerical discrepancy.

**Changes:** 

- Modified from "two kinds of servers" to "three kinds of servers". 
- File: `docs/mcp.md` (line 11).

Author: luochang212

docs/mcp.md

@@ -8,7 +8,7 @@ The Agents SDK has support for MCP. This enables you to use a wide range of MCP
 
 ## MCP servers
 
-Currently, the MCP spec defines two kinds of servers, based on the transport mechanism they use:
+Currently, the MCP spec defines three kinds of servers, based on the transport mechanism they use:
 
 1. **stdio** servers run as a subprocess of your application. You can think of them as running "locally".
 2. **HTTP over SSE** servers run remotely. You connect to them via a URL.