(svn r14105) -Fix: Some typos that are more grammatical errors, as it seems.

Provided by ln
I've added a little fix on currency.cpp too, why not :)

src/misc/binaryheap.hpp

@@ -25,7 +25,7 @@
  *    use indices 1..max_items instead of zero based C indexing.
  *
  * 3) Item of the binary heap should support these public members:
- *    - 'lower-then' operator '<' - used for comparing items before moving
+ *    - 'lower-than' operator '<' - used for comparing items before moving
  *
  */