Fix unknown directive warnings in c++-mode/class11

[shassard]

trafficstars

The following warnings are emitted by yasnippet:

Ignoring unknown directive "desc" in file: .../yasnippet-snippets/snippets/c++-mode/class11
Ignoring unknown directive "\\${[0-9N]*" in file: .../yasnippet-snippets/snippets/c++-mode/class11
Ignoring unknown directive "References" in file: .../yasnippet-snippets/snippets/c++-mode/class11

Use an unconventional ## to mark comments so yasnippet will ignore lines that contain colons.