Mention at $< what the user should do if he instead wants the 2nd

[積丹尼 Dan Jacobson]

On (info "(make) Automatic Variables") after

'$<'
     The name of the first prerequisite.  If the target got its recipe
     from an implicit rule, this will be the first prerequisite added by
     the implicit rule (*note Implicit Rules::).

Please add:

     For instead e.g., the fourth prerequisite, use $(word 4,$^). For
     e.g., the seventh through ninth, use $(wordlist 7,9,$^).

Thanks.