Use the normalized file names in doc/.

The script that looks for code gallery programs wants files without .txt ending,
but this directory had them with a file ending. Fix this.

diff --git a/Nonlinear_PoroViscoelasticity/doc/author.txt b/Nonlinear_PoroViscoelasticity/doc/author
similarity index 100%
rename from Nonlinear_PoroViscoelasticity/doc/author.txt
rename to Nonlinear_PoroViscoelasticity/doc/author

diff --git a/Nonlinear_PoroViscoelasticity/doc/builds-on.txt b/Nonlinear_PoroViscoelasticity/doc/builds-on
similarity index 100%
rename from Nonlinear_PoroViscoelasticity/doc/builds-on.txt
rename to Nonlinear_PoroViscoelasticity/doc/builds-on

diff --git a/Nonlinear_PoroViscoelasticity/doc/dependencies.txt b/Nonlinear_PoroViscoelasticity/doc/dependencies
similarity index 100%
rename from Nonlinear_PoroViscoelasticity/doc/dependencies.txt
rename to Nonlinear_PoroViscoelasticity/doc/dependencies

diff --git a/Nonlinear_PoroViscoelasticity/doc/entry-name.txt b/Nonlinear_PoroViscoelasticity/doc/entry-name
similarity index 100%
rename from Nonlinear_PoroViscoelasticity/doc/entry-name.txt
rename to Nonlinear_PoroViscoelasticity/doc/entry-name

diff --git a/Nonlinear_PoroViscoelasticity/doc/tooltip.txt b/Nonlinear_PoroViscoelasticity/doc/tooltip
similarity index 100%
rename from Nonlinear_PoroViscoelasticity/doc/tooltip.txt
rename to Nonlinear_PoroViscoelasticity/doc/tooltip