HttpMessageConverters import is wrong in sample code in the reference docs

[wilkinsona]

Via Stack Overflow, the import is wrong in the sample code in the reference documentation.

[snicoll]

for the record I meant to move this one to a code snippet that we could compile but the fact it's pseudo-code at the moment makes it quite hard for that kind of setup. We can reopen if we think that we're missing an opportunity to align.