fixed Pair method fmt it was missing the format sting in the signature

Author: grizzlysmit

doc/Type/Pair.pod6

@@ -84,7 +84,7 @@ I<key> parts, and then compares the I<value> parts if the keys are equal.
 
 Defined as:
 
-    multi method fmt(Pair:D:) returns Str:D
+    multi method fmt(Pair:D: Str:D $format) returns Str:D
 
 Takes a I<format string>, and returns a string the I<key> and I<value>
 parts of the C<Pair> formatted. Here's an example: