CONTRIBUTING.rst: Update for 120 column limit

Commit a1fbf1015f40 updated .clang-format and existing code for 120
columns, so change Coding Guidelines to match.

CONTRIBUTING.rst

@@ -54,7 +54,7 @@ Coding Guidelines
 - Tabs for indentation, spaces for alignment.  Tabs are treated as 8
   columns wide.
 
-- 80 columns max
+- 120 columns max
 
 - Comments and names of variables/functions/etc. must be in English