fix: typo MCP_SESSION_ID, keep consistent style with other identifiers (#564)

Author: JunJieLiu51520

mcp-core/src/main/java/io/modelcontextprotocol/spec/HttpHeaders.java

@@ -14,7 +14,7 @@ public interface HttpHeaders {
 	/**
 	 * Identifies individual MCP sessions.
 	 */
-	String MCP_SESSION_ID = "mcp-session-id";
+	String MCP_SESSION_ID = "Mcp-Session-Id";
 
 	/**
 	 * Identifies events within an SSE Stream.