doc: add missing initialisation in example code. Thanks to Satoshi Ohshima for reporting the error

doc/starpu.texi

@@ -789,7 +789,7 @@ driver.
 static __global__ void vector_mult_cuda(float *val, unsigned n,
                                         float factor)
 @{
-    unsigned i;
+    unsigned i = threadIdx.x;
     if (i < n)
         val[i] *= factor;
 @}

doc/vector_scal_cuda.texi

@@ -3,7 +3,7 @@
 static __global__ void vector_mult_cuda(float *val, unsigned n,
                                         float factor)
 @{
-        unsigned i;
+        unsigned i = threadIdx.x;
         if (i < n)
                val[i] *= factor;
 @}