Bump version of clang format down one for Debian

CONTRIBUTING.md

@@ -150,8 +150,8 @@ file to allow for automatic code formatting. As noted above, you can use this
 for new files, or cosmetic `tidy/*` branches, but try to stick to existing
 formatting elsewhere.
 
-Installing it is easy on most package managers. Nektar++ relies on options that
-are used in version 3.7 or later.
+Installing it is straightforward on most package managers. Nektar++ relies on
+options that are used in version 3.6 or later.
 
 There are a number of instructions on how to use `clang-format` inside a number
 of text editors on the