Fixed typo in example

It should be `css_class` in the template tag example instead of just `class`. (It is mentioned correctly in the syntax line earlier).
Merge pull request #1025 from reinout/patch-1

Fixed typo in templatetag docs example (thanks to @reinout)
Fixed the installing search engine doc URL
Link to 'signal processors'
Merge pull request #1047 from g3rd/patch-1

Update the installing search engine documentation URL (thanks @g3rd)
Merge pull request #1050 from jogwen/patch-2

Documentation: add a link to signal processors in search index
Docs: fix signal processors link in searchindex_api

Correct a typo in b676b17dbc4b29275a019417e7f19f531740f05e