user-manual: fix incorrect header level

This section is a subsection of the "Examples" section.
Signed-off-by: NJ. Bruce Fields <bfields@citi.umich.edu>

Documentation/user-manual.txt

@@ -914,7 +914,7 @@ they look OK.
 
 [[Finding-comments-with-given-content]]
 Finding commits referencing a file with given content
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+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Somebody hands you a copy of a file, and asks which commits modified a
 file such that it contained the given content either before or after the