CeylonLexer: Fix backticks in string templates

For a sequence like

    "This is ```module ceylon.lexer```"

two separations into tokens are possible:

    "This is ``  `  module  ceylon  .  lexer  ` ``"
    "This is ```  module  ceylon  .  lexer  ```"

In the latter case, the string parts each contain one backtick.

The RedHat compiler chooses the latter variant, and so should we.