Bad substitution: no closing "`" in a heredoc / EOF

The backtick introduces a command substitution. Since the here-document is not quoted, this will be interpreted by the shell. The shell complains since the command substitution has no ending backtick.

To quote a here-document, use

cat <<'END_HELP'
something something help
END_HELP

or

cat <<\END_HELP
something something help
END_HELP

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Regarding your comments on the resolution of this issue:

Utilities seldom output a complete manual by themselves but may offer a synopsis or basic usage information. This is seldom, if ever, colorized (since its output may not be directed to a terminal or pager like less). The real manual is often typesetted using groff or a dedicated man-page formatter like mandoc and is handled completely separate from the code.

You want to specifically use a backtick/grave accent with the single quote/apostrophe to quote something? Please don't, the combination looks awful. In most fonts, the backtick is slanted, and the (ASCII) apostrophe is straight. This is how my browser shows the last line of your man page snippet:

If you want to use quotes that are fancier than the vertical ASCII quotes, you should probably use something like U+2018 and U+2019.

The output would of course depend on your fonts, but I think ‘this’ looks better than `this'.