src/doc/en/thematic_tutorials/vector_calculus: add long times

Fix the CI warnings,

  2025-07-14T18:26:34.0468072Z ##[warning]slow doctest:
  2025-07-14T18:26:34.0476899Z     spherical.plot(cartesian, color={r:'red', th:'green', ph:'orange'})
  2025-07-14T18:26:34.0478151Z Test ran for 5.28s cpu, 5.37s wall

  2025-07-14T18:26:51.7069355Z ##[warning]slow doctest:
  2025-07-14T18:26:51.7074158Z     Du = laplacian(u)
  2025-07-14T18:26:51.7078276Z Test ran for 7.21s cpu, 7.28s wall

by adding "# long time" to these tests.

Author: orlitzky

src/doc/en/thematic_tutorials/vector_calculus/vector_calc_change.rst

@@ -138,6 +138,7 @@ the above call ``E.spherical_coordinates()``::
 These formulas are automatically used if we ask to plot the grid of spherical
 coordinates in terms of Cartesian coordinates::
 
+    sage: # long time
     sage: spherical.plot(cartesian, color={r:'red', th:'green', ph:'orange'})
     Graphics3d Object
 

src/doc/en/thematic_tutorials/vector_calculus/vector_calc_curvilinear.rst

@@ -232,6 +232,7 @@ The Laplacian of a scalar field::
 
 The Laplacian of a vector field::
 
+    sage: # long time
     sage: Du = laplacian(u)
     sage: Du.display()
     Delta(u) = ((r^2*d^2(u_r)/dr^2 + 2*r*d(u_r)/dr - 2*u_r(r, th, ph)
@@ -247,7 +248,7 @@ The Laplacian of a vector field::
 Since this expression is quite lengthy, we may ask for a display component by
 component::
 
-    sage: Du.display_comp()
+    sage: Du.display_comp()  # long time
     Delta(u)^1 = ((r^2*d^2(u_r)/dr^2 + 2*r*d(u_r)/dr - 2*u_r(r, th, ph) + d^2(u_r)/dth^2
      - 2*d(u_theta)/dth)*sin(th)^2 - ((2*u_theta(r, th, ph) - d(u_r)/dth)*cos(th)
      + 2*d(u_phi)/dph)*sin(th) + d^2(u_r)/dph^2)/(r^2*sin(th)^2)
@@ -260,6 +261,7 @@ component::
 
 We may expand each component::
 
+    sage: # long time
     sage: for i in E.irange():
     ....:     s = Du[i].expand()
     sage: Du.display_comp()