Few typos

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@19685 b5f69653-2bf5-0310-8201-b0dafb8e1405
#2385 Correct estimation of non-zeroes for  PETSc 3.3 and 3.4

git-svn-id: https://chaste.cs.ox.ac.uk/svn/chaste/trunk@19688 b5f69653-2bf5-0310-8201-b0dafb8e1405