docs(examples): correct use of GetArguments (#316)

As part of 28c9cc3 we made
`Params.Arguments` return an `any`.

We'd updated almost all the documentation, aside from this case.

This doesn't impact the Prompt examples, as they're still using the old
method.

Co-authored-by: Navendu Pottekkat <navendu@apache.org>

Author: jamietanna

README.md

@@ -58,7 +58,7 @@ func main() {
 }
 
 func helloHandler(ctx context.Context, request mcp.CallToolRequest) (*mcp.CallToolResult, error) {
-    name, ok := request.Params.Arguments["name"].(string)
+    name, ok := request.GetArguments()["name"].(string)
     if !ok {
         return nil, errors.New("name must be a string")
     }