Fix problem that field symbol definitions are not visible

[tekand]

They are treated as HTML tags when inside of <pre>. Changing the syntax highlighting to triple ticks. Making syntax highlighting consistent, ABAP vs ABAP-CDS.

Example:
Exercise 6.1.2: Implement the Action Method

Field symbol definition shows up as:

LOOP AT travels ASSIGNING FIELD-SYMBOL().  
      DATA(reduced_fee) = -BookingFee * ( 1 - 3 / 10 ) .  

      APPEND VALUE #( %tky       = -%tky
                    BookingFee = reduced_fee
                  ) TO travels_for_update.
ENDLOOP.

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