change name of marketplace in opensearch; bug 1044290

you need to change [the test below](#diff-f6ea3f5b1c38b147cd9d2de1d7bfee8eR156) too:

```
======================================================================
FAIL: test_opensearch (zamboni.mkt.site.tests.test_views.TestOpensearch)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/opt/zamboni/mkt/site/tests/test_views.py", line 162, in test_opensearch
    eq_(e.text, 'Firefox Marketplace')
AssertionError: 'Firefox Apps' != 'Firefox Marketplace'

----------------------------------------------------------------------
Ran 18 tests in 2.126s

FAILED (failures=1)
```