Reword renamed-buffer prompt to be more clear

[aflock]

When renaming a file that is currently open in a buffer, NerdTree helpfully asks us:

Node renamed. The old file is open in buffer <buf>. Replace this buffer with *a* new file? (yN)

(emphasis mine)

This is slightly confusing as replacing the buffer with a new file isn't what is meant, instead we can be more clear just by saying 'the new file', the one we just renamed to.

Node renamed. The old file is open in buffer <buf>. Replace this buffer with the new file? (yN)

[PhilRunninger]

Good catch. Thanks for the contribution.