Remove backslash from Twitter handle

Author: afh

source/contribute.html.md

@@ -27,4 +27,4 @@ The [ledger source code](http://git.ledger-cli.org/) is available on GitHub, con
 **Twitter**  
 [#ledgercli](https://twitter.com/search?q=%23ledgercli&src=typd&f=realtime) Ledger-related tweets  
 [#plaintextaccounting](https://twitter.com/search?q=%23ledgercli&src=typd&f=realtime) plain text accounting in general  
-[\@LedgerTips](https://twitter.com/ledgertips) tips for Ledger, hledger, beancount and similar  
+[@LedgerTips](https://twitter.com/ledgertips) tips for Ledger, hledger, beancount and similar