fix Markdown in the Lockable doc string

Author: nsajko

base/lock.jl

@@ -398,7 +398,7 @@ macro lock_nofail(l, expr)
 end
 
 """
-  Lockable(value, lock = ReentrantLock())
+    Lockable(value, lock = ReentrantLock())
 
 Creates a `Lockable` object that wraps `value` and
 associates it with the provided `lock`. This object