sage.tests.book_schilling_zabrocki_kschur_primer: add one "long time"

The line,

  sage: G1 = SymQ3.AffineGrothendieckPolynomial([1],6)

is triggering slow doctest warnings on the CI, and takes over 50s to
complete on my (slow) machine. Let's mark it "long time" to avoid the
warnings.

Author: orlitzky

src/sage/tests/book_schilling_zabrocki_kschur_primer.py

@@ -786,6 +786,7 @@
 
 Sage example in ./kschurnotes/notes-mike-anne.tex, line 5588::
 
+sage: # long time
 sage: SymQ3 = Sym.kBoundedQuotient(3,t=1)
 sage: G1 = SymQ3.AffineGrothendieckPolynomial([1],6)
 sage: G2 = SymQ3.AffineGrothendieckPolynomial([2],6)