Clarify lambda interface to map.upsert (ponylang#1363)

The current example is unclear as to which value is the old value
and which is the new value when setting up the lambda. By switching
from addition to subtraction in the example, this becomes clear.

This documentation could be improved further but this is a decent
start towards resolving that possible confusion without having
to look at the source code.

Author: SeanTAllen

packages/collections/map.pony

@@ -87,19 +87,19 @@ class HashMap[K, V, H: HashFunction[K] val]
     using the provided function. If the provided key has not been added to
     the map yet, it sets its value to the provided value.
 
-    As a simple example, say we had a map with U64 values and we wanted to
+    As a simple example, say we had a map with I64 values and we wanted to
     add 4 to the current value for key "test", which let's say is currently 2.
     We call
 
-    m.upsert("test", 4, lambda(x: U64, y: U64): U64 => x + y end)
+    m.upsert("test", 4, lambda(x: I64, y: I64): I64 => x - y end)
 
-    This changes the value associated with "test" to 6.
+    This changes the value associated with "test" to -2.
 
     If we have not yet added the key "new-key" to the map and we call
 
-    m.upsert("new-key", 4, lambda(x: U64, y: U64): U64 => x + y end)
+    m.upsert("new-key", 4, lambda(x: I64, y: I64): I64 => x - y end)
 
-    then "new-key" is added to the map with a value of 4.
+    then "new-key" is added to the map with a value of -4.
 
     Returns the value that we set the key to
     """