Added string specifier in format strings (#3)

This is now required in current versions of Zig.

exercises/06_strings.zig

@@ -35,11 +35,12 @@ pub fn main() void {
     // That's all the problems. Let's see our results:
     std.debug.print("d={u} {s}{s}\n",.{d, laugh, major_tom});
     //
-    // Keen eyes will notice that we've put a 'u' inside the '{}'
-    // placeholder in the format string above. This tells the
-    // print() function to format the values as a UTF-8 character.
-    // If we didn't do this, we'd see '100', which is the decimal
-    // number corresponding with the 'd' character in UTF-8.
+    // Keen eyes will notice that we've put 'u' and 's' inside the '{}'
+    // placeholders in the format string above. This tells the
+    // print() function to format the values as a UTF-8 character and
+    // UTF-8 strings respectively. If we didn't do this, we'd see '100',
+    // which is the decimal number corresponding with the 'd' character
+    // in UTF-8. (And an error in the case of the strings.)
     //
     // While we're on this subject, 'c' (ASCII encoded character)
     // would work in place for 'u' because the first 128 characters

exercises/07_strings2.zig

@@ -20,5 +20,5 @@ pub fn main() void {
         And the Spiders from Mars
     ;
 
-    std.debug.print("{}\n",.{lyrics});
+    std.debug.print("{s}\n",.{lyrics});
 }

exercises/08_quiz.zig

@@ -30,5 +30,5 @@ pub fn main() void {
     lang[2] = letters[???];
 
     // We want to "Program in Zig!" of course:
-    std.debug.print("Program in {}!\n", .{lang});
+    std.debug.print("Program in {s}!\n", .{lang});
 }