Fix typo in new-in-3.2.xml document

This commit fixes a typo in the "New Features and Enhancements in Spring
Framework 3.2" chapter of the reference manual. Specifically,
ContentNegotiationStrategy is now spelled correctly.
Merge pull request #221 from kdombeck/fix-typo

# By Ken Dombeck
* fix-typo:
  Fix typo in new-in-3.2.xml document