Add type constrains to speed up examples

MasterDuke17++

It should be nice if keep using effective code in examples.

Author: tisonkun

doc/Language/regexes.pod6

@@ -1264,10 +1264,10 @@ There are four ways you can interpolate a string into regex as a pattern.
 That is using C«$pattern», C«$($pattern)», C«<$pattern>» or
 C«<{$pattern.method}>».
 
-    my $text = 'camelia';
-    my $pattern0 = 'camelia';
-    my $pattern1 = 'ailemac';
-    my $pattern2 = '\w+';
+    my Str $text = 'camelia';
+    my Str $pattern0 = 'camelia';
+    my Str $pattern1 = 'ailemac';
+    my Str $pattern2 = '\w+';
 
     say $text ~~ / $pattern0 /;                # OUTPUT: «｢camelia｣␤»
     say $text ~~ / $($pattern0) /;             # OUTPUT: «｢camelia｣␤»