git-diff: update description

Replace backticks with quotes

pages/common/git-diff.md

@@ -14,7 +14,7 @@
 
 `git diff --staged`
 
-- Show changes from all commits since a given date/time (a date expression, e.g. `1 week 2 days` or an ISO date):
+- Show changes from all commits since a given date/time (a date expression, e.g. "1 week 2 days" or an ISO date):
 
 `git diff 'HEAD@{3 months|weeks|days|hours|seconds ago}'`