llama-bench: fixed help output for -override-tensors to -override-tensor

[jukofyork]

Just fixed this help output for llama-bench

  -ot --override-tensors <tensor name pattern>=<buffer type>;...
                                            (default: disabled)

error: invalid parameter for argument: --override-tensors

it should be --override-tensor to match the rest of llama.cpp.

[4onen]

Whoops, sorry about that.