fixed typo in common/content/tabs.js
fixed typo in common/content/tabs.js

line 428, borwser -> browser
Fixes issue 896

Evidlo/vimperator-labs,Quicksaver/vimperator-labs,Evidlo/vimperator-labs,airodactyl/airodactyl,Sheile/vimperator-labs,airodactyl/airodactyl,Quicksaver/vimperator-labs,nullren/vimperator-labs,paretje/vimperator-labs,zklinger/vimperator-labs,seanmorton/vimperator-labs,seanmorton/vimperator-labs,derenio/vimperator-labs,stemar94/vimperator-labs,zklinger/vimperator-labs,nullren/vimperator-labs,derenio/vimperator-labs,mcarton/vimperator-labs,stemar94/vimperator-labs,Sheile/vimperator-labs,mcarton/vimperator-labs,paretje/vimperator-labs