doxygen: address unresolved reference warnings in range.hpp

[eramongodb]

A small followup to 0f990ca to address the following Doxygen warning:

src/mongocxx/include/mongocxx/v_noabi/mongocxx/options/range.hpp:36: warning: unable to resolve reference to 'trimFactor' for \ref command

Demotes mentions of encryptedFields members to inline monospace text rather than ambiguous or non-existent references to Doxygen entities. (Note: the valid references happen to link to the getter overload for each member function.)

Before:

After:

[kevinAlbs]

Oop. Thank you for the fix.