s/head/first/ & s/tail/rest/ in a testcase

Makes the testcase in sync with example grammars.

spec/generated-parser.spec.js

@@ -1115,19 +1115,19 @@ describe("generated parser", function() {
          */
         var parser = PEG.buildParser([
               'Expr    = Sum',
-              'Sum     = head:Product tail:(("+" / "-") Product)* {',
-              '            var result = head, i;',
-              '            for (i = 0; i < tail.length; i++) {',
-              '              if (tail[i][0] == "+") { result += tail[i][1]; }',
-              '              if (tail[i][0] == "-") { result -= tail[i][1]; }',
+              'Sum     = first:Product rest:(("+" / "-") Product)* {',
+              '            var result = first, i;',
+              '            for (i = 0; i < rest.length; i++) {',
+              '              if (rest[i][0] == "+") { result += rest[i][1]; }',
+              '              if (rest[i][0] == "-") { result -= rest[i][1]; }',
               '            }',
               '            return result;',
               '          }',
-              'Product = head:Value tail:(("*" / "/") Value)* {',
-              '            var result = head, i;',
-              '            for (i = 0; i < tail.length; i++) {',
-              '              if (tail[i][0] == "*") { result *= tail[i][1]; }',
-              '              if (tail[i][0] == "/") { result /= tail[i][1]; }',
+              'Product = first:Value rest:(("*" / "/") Value)* {',
+              '            var result = first, i;',
+              '            for (i = 0; i < rest.length; i++) {',
+              '              if (rest[i][0] == "*") { result *= rest[i][1]; }',
+              '              if (rest[i][0] == "/") { result /= rest[i][1]; }',
               '            }',
               '            return result;',
               '          }',