doc: Periodic spellcheck of the documentation. * doc/interpreter/doccheck/aspell-octave.en.pws: Update Octave-specific Aspell dictionary. * doc/interpreter/contrib.txi, doc/interpreter/errors.txi, doc/interpreter/expr.txi, doc/interpreter/io.txi, doc/interpreter/preface.txi, doc/interpreter/sparse.txi, doc/interpreter/stmt.txi, doc/interpreter/strings.txi, libinterp/corefcn/qz.cc, libinterp/dldfcn/ccolamd.cc, libinterp/dldfcn/fftw.cc, libinterp/interpfcn/graphics.cc, scripts/general/structfun.m, scripts/image/ind2rgb.m, scripts/image/rgb2ind.m, scripts/io/importdata.m, scripts/linear-algebra/duplication_matrix.m, scripts/linear-algebra/housh.m, scripts/miscellaneous/mkoctfile.m, scripts/plot/comet3.m, scripts/plot/gco.m, scripts/plot/legend.m, scripts/plot/print.m, scripts/signal/fftshift.m, scripts/signal/periodogram.m, scripts/sparse/pcg.m, scripts/statistics/tests/f_test_regression.m, scripts/statistics/tests/t_test_regression.m, scripts/strings/strjoin.m, scripts/strings/strsplit.m: Correct spellings or add @nospell macro.

--- a/scripts/linear-algebra/duplication_matrix.m
+++ b/scripts/linear-algebra/duplication_matrix.m
@@ -23,7 +23,7 @@
 ##  $D_n$
 ## @end tex
 ## @ifnottex
-##  @math{Dn}
+##  @nospell{@math{Dn}}
 ## @end ifnottex
 ##  which is the unique
 ## @tex
@@ -37,7 +37,7 @@
 ##  $D_n * {\rm vech} (A) = {\rm vec} (A)$
 ## @end tex
 ## @ifnottex
-##  @math{Dn vech (A) = vec (A)}
+##  @nospell{@math{Dn vech (A) = vec (A)}}
 ## @end ifnottex
 ##  for all symmetric
 ## @tex