fix: correct parameter name in FastMCP initialization

[arno4000]

This pull request fixes the initialization of the FastMCP server in main.py, replacing the use of the description parameter with instructions.

Without this change, the MCP server is unable to start

Traceback (most recent call last):
  File "/workdir/main.py", line 15, in <module>
    mcp = FastMCP("itop-mcp", description="iTop ITSM integration server")
TypeError: FastMCP.__init__() got an unexpected keyword argument 'description'