Update trigger description in README.md

The number of triggers is configurable since https://github.com/riscv-software-src/riscv-isa-sim/pull/1219

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The trigger description was for the limited implementation when developing the extension. All trigger types are supported now. The information isn't very useful anymore.

Signed-off-by: YenHaoChen <39526191+YenHaoChen@users.noreply.github.com>