Update variables.pod6

constant defined with prefixes but not in summary table

doc/Language/variables.pod6

@@ -460,10 +460,11 @@ There are also two prefixes that resemble declarators but act on
 predefined variables:
 
 =for table
-    Prefix  Effect
-    ======  ======
-    temp    Restores a variable's value at the end of scope
-    let     Restores a variable's value at the end of scope if the block exits unsuccessfully
+    Prefix    Effect
+    ======    ======
+    temp      Restores a variable's value at the end of scope
+    let       Restores a variable's value at the end of scope if the block exits unsuccessfully
+    constant  Declares that a container value is not going to change during its lifetime
 
 =head2 The C<my> declarator