Docs: Correct the default random engine in GATE

The default GATE random engine is James Random (and apparently has been
throughout the available Git history), but the documentation mistakenly stated
MersenneTwister as the default.

docs/getting_started.rst

@@ -627,7 +627,7 @@ time duration, the last line of your program has to be **exit**.
 As a Monte Carlo tool, GATE needs a random generator. The CLHEP libraries
 provide various ones. Four different random engines are currently available in
 GATE, Ranlux64, James Random, MixMaxRng (default random engine in Geant4), and Mersenne Twister. The default random
-engine in GATE is Mersenne Twister, but this can be changed easily using::
+engine in GATE is James Random, but this can be changed easily using::
 
   /gate/random/setEngineName aName    (where aName can be: Ranlux64, JamesRandom, MixMaxRng, or MersenneTwister)