[commands] Elaborate more on disallowed types in Greedy and Optional

docs/ext/commands/api.rst

@@ -185,7 +185,7 @@ Converters
     As a consequence of this behaviour, most input errors are silently discarded,
     since it is used as an indicator of when to stop parsing.
 
-    When a parser error is met the greedy converter stops converting, it undos the
+    When a parser error is met the greedy converter stops converting, undoes the
     internal string parsing routine, and continues parsing regularly.
 
     For example, in the following code: