Help msg about context shift is incorrect

[DamonFool]

Help msg about context shift is incorrect

After #15416 , context shift had been disabled by default.
However, the help msg is incorrect.

For example, if we run llama-cli -h

--no-context-shift                      disables context shift on infinite text generation (default: enabled)
                                        (env: LLAMA_ARG_NO_CONTEXT_SHIFT)
--context-shift                         enables context shift on infinite text generation (default: enabled)
                                        (env: LLAMA_ARG_CONTEXT_SHIFT)

After this patch, it would be

--no-context-shift                      disables context shift on infinite text generation (default: enabled)
                                        (env: LLAMA_ARG_NO_CONTEXT_SHIFT)
--context-shift                         enables context shift on infinite text generation (default: disabled)
                                        (env: LLAMA_ARG_CONTEXT_SHIFT)

The patch also fixes a typo about ctx_shift (inifinite --> infinite).

[DamonFool]

Thanks @ggerganov .