Replace 'should' with 'must'

Author: mcol

src/functions-reference/sparse_matrix_operations.Rmd

@@ -97,7 +97,7 @@ entries w, column indices v, and row starting indices u; the vector w
 and array v must be the same size (corresponding to the total number of
 nonzero entries in the matrix), array v must have index values bounded
 by m, array u must have length equal to m + 1 and contain index values
-bounded by the number of nonzeros (except for the last entry, which should
+bounded by the number of nonzeros (except for the last entry, which must
 be equal to the number of nonzeros plus one). See section
 [compressed row storage](#CSR) for more details.