Fix lammps.book and broken link

doc/src/Manual.txt

@@ -49,7 +49,7 @@ all LAMMPS development is coordinated.
 
 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.

doc/src/lammps.book

@@ -127,7 +127,7 @@ Errors_common.html
 Errors_bugs.html
 Errors_messages.html
 Errors_warnings.html
-Build_manual.html
+Manual_build.html
 
 lammps_commands.html
 atom_modify.html