Remove outdated remark in documentation of ContextDefinition

The documentation for ContextDefinition mentions the following:

Remove spaces after @ in your actual plugin - these are put into this sample code so that it is not recognized as an annotation.

However there are no spaces after the @ symbol. I tracked this down and this remark was introduced in the original issue #2281635: \Drupal\Core\Plugin\Context\Context should use a ContextDefinition class instead of arrays in comment #31. The spaces after the @ symbol were removed again in comment #38 but this was not spotted in subsequent reviews.