Update share.md

Fix grammar.

# Why
"a" should be used instead of "an" before the word "URL" because the U here is a consonant sound. (e.g. a Uniform)

Author: walkeknow

docs/share.md

@@ -116,7 +116,7 @@ In Android, returns a Promise which will always be resolved with action being `S
 
 #### iOS
 
-- `url` - an URL to share
+- `url` - a URL to share
 
 At least one of URL and message is required.