Smal doc update.

doc/notebooks/vsop2013.ipynb

@@ -210,7 +210,7 @@
     "\n",
     "A possible issue in astrodynamical applications is that VSOP2013 does not directly provide formulae for the state vector of the Sun. The barycentric position of the Sun can be estimated via the heliocentric planetary formulae after imposing that the system's barycentre is fixed in the origin. It is not clear at this time if this approach is 100% accurate (as VSOP2013 includes perturbations on the planets by asteroids, whose state vectors are not provided in the solution), but it should be adequate for most practical purposes.\n",
     "\n",
-    "A better alternative is to formulate the dynamical equations of motion (e.g., of a spacecraft or a small body) in a heliocentric reference frame (i.e., in the framework of the so-called (N+1)-body problem)."
+    "Another option is to formulate the dynamical equations of motion (e.g., of a spacecraft or a small body) in a heliocentric reference frame (i.e., in the framework of the so-called (N+1)-body problem)."
    ]
   }
  ],