Fix link formatting error

Had a newline in middle of a link, causing the link text to be shown with `[]`.

Author: brettcannon

src/doc/trpl/installing-rust.md

@@ -90,8 +90,8 @@ If not, there are a number of places where you can get help. The easiest is
 [the #rust IRC channel on irc.mozilla.org][irc], which you can access through
 [Mibbit][mibbit]. Click that link, and you'll be chatting with other Rustaceans
 (a silly nickname we call ourselves), and we can help you out. Other great
-resources include [the user’s forum][users], and [Stack Overflow][stack
-overflow].
+resources include [the user’s forum][users], and
+[Stack Overflow][stack overflow].
 
 [irc]: irc://irc.mozilla.org/#rust
 [mibbit]: http://chat.mibbit.com/?server=irc.mozilla.org&channel=%23rust