ports.7: Fix example of passing variables on the command line

make(1)'s -D flag does not allow for setting the value of the variable.
It just defines the variable and sets its value to 1. In fact, make(1)
treats "=" as just another character in the variable name:

$ make -DA=2 -V A  # Output is just an empty line.

$ make -DA=2 -V A=2  # Variable "A=2" is defined and set to "1".
1

Fixes: d25f7d324a9d ports.7: Document DEBUG_FLAGS and the process of debugging ports
MFC after: 3 days