Fix lammps.book and broken link

The content for this manual is part of the LAMMPS distribution.

You can build a local copy of the Manual as HTML pages or a PDF file,

by following the steps on the "this page"_Build_manual.html.

by following the steps on the "this page"_Manual_build.html.

There is also a "Developer.pdf"_Developer.pdf document which gives

a brief description of the basic code structure of LAMMPS.

Errors_bugs.html

Errors_messages.html

Errors_warnings.html

Build_manual.html

Manual_build.html

lammps_commands.html

atom_modify.html