Replace vars with consts in async exercises

exercises/088_async5.zig

@@ -36,7 +36,7 @@ const print = @import("std").debug.print;
 pub fn main() void {
     var myframe = async getPageTitle("http://example.com");
 
-    var value = ???
+    const value = ???
 
     print("{s}\n", .{value});
 }

exercises/089_async6.zig

@@ -41,8 +41,8 @@ pub fn main() void {
     var com_frame = async getPageTitle("http://example.com");
     var org_frame = async getPageTitle("http://example.org");
 
-    var com_title = com_frame;
-    var org_title = org_frame;
+    const com_title = com_frame;
+    const org_title = org_frame;
 
     print(".com: {s}, .org: {s}.\n", .{ com_title, org_title });
 }

exercises/090_async7.zig

@@ -29,7 +29,7 @@ pub fn main() void {
     // The main() function can not be async. But we know
     // getBeef() will not suspend with this particular
     // invocation. Please make this okay:
-    var my_beef = getBeef(0);
+    const my_beef = getBeef(0);
 
     print("beef? {X}!\n", .{my_beef});
 }