DEV: Correct typo in README (#412)

DEV: Correct typo in README (#412)

“its” possessive: not “it is”

diff --git a/README.md b/README.md
index cdf7fd7..2d8810c 100644
--- a/README.md
+++ b/README.md
@@ -184,7 +184,7 @@ Adding Support for a new URL
 Whitelisted Generic Onebox caveats
 ----------------------------------
 
-The Whitelisted Generic Onebox has some caveats for it's use, beyond simply whitelisting the domain.
+The Whitelisted Generic Onebox has some caveats for its use, beyond simply whitelisting the domain.
 
   1. The domain must be whitelisted
   2. The URL you're oneboxing cannot be a root url (e.g. `http://example.com` won't work, but `http://example.com/page` will)

GitHub sha: bca054ec