docs: mention invalid TERM ncurses error

[raravind007]

What changed:

• Added a troubleshooting note for ncurses failures when $TERM is unset or invalid.

Why:

• nvtop may exit with "ncurses: cannot initialize terminal type" in sandboxed or minimal terminals when $TERM is unset or invalid.
• Setting $TERM to a valid terminal type, such as xterm-256color, resolves this.

Testing:

• Built nvtop locally on Fedora 44.
• Verified the error occurred with TERM=unknown and was resolved with TERM=xterm-256color locally.