Fix "covarage" typo in XSL transforms

To the best of my knowledge, no actual XML format requires the
"covarage" typo as an element or attribute, and no site should be
entering a "covarage" metadata field.

Signed-off-by: Dan Scott <dan@coffeecode.net>
Merge pull request #581 from dbs/coverage_typo

DS-2073 - Fix "covarage" typo in XSL transforms (OAI-PMH Crosswalks)