Removing duplicate boto.s3.prefix entry to prevent Sphinx build errors.

This change removes a redundant `boto.s3.prefix` reference (note, the reference already exists on line 56), thus eliminating the following documentation build problem in Sphinx v0.6.4. 
```
$ make html
sphinx-build -b html -d build/doctrees   source build/html
Making output directory...
Running Sphinx v0.6.4
HEAD
Adding github link roles
loading pickled environment... not found
WARNING: intersphinx inventory 'http://docs.python.org/objects.inv' not readable due to <type 'exceptions.ValueError'>: unknown or unsupported inventory version
building [html]: targets for 96 source files that are out of date
updating environment: 96 added, 0 changed, 0 removed
reading sources... [ 51%] ref/s3                                                                                              
reST markup error:
/home/pingwin/src/boto/docs/source/ref/s3.rst:: (SEVERE/4) Duplicate ID: "module-boto.s3.prefix".
make: *** [html] Error 1
```