docs: fix analytical bandwidth parameters

[leno23]

Closes #227.

Summary

• replace the unsupported -busbw analytical example with the currently parsed -nv, -nic, and -n_p_s parameters
• update the analytical mode description to avoid referring to a bus bandwidth file option
• fix the help text for -n_p_s so it documents --nic_per_server

Verification

• git diff --check
• rg -n -- "-busbw|--bus-bandwidth" README.md astra-sim-alibabacloud/astra-sim/system/AstraParamParse.hh

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