ref-manual: Corrected the "package_rpm.bbclass" section.

A cut-and-paste error had left a "package_deb" string in the
first sentence of the section.  Replaced with "package_rpm."

Reported-by: Geoffroy VanCutsem <geoffroy.vancutsem@intel.com>
(From yocto-docs rev: 38b20d775ba41cb14d86177dddd85ada32fb3f86)

Signed-off-by: Scott Rifenbark <scott.m.rifenbark@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>