docs: Fix gulp task causing duplicate demo files.

If two demo directories began with the same name (like demoErrors
and demoErrorsAdvanced), the gulp task would miscalculate the
number and type of files causing duplicate files to appear in the
source view and Codepen examples.

Fix by ensuring we only match the proper directory.

  Closes angular#6254

scripts/gulp-utils.js

@@ -63,7 +63,7 @@ exports.readModuleDemos = function(moduleName, fileTasks) {
         css:[], html:[], js:[]
       };
 
-      gulp.src(demoFolder.path + '**/*', { base: path.dirname(demoFolder.path) })
+      gulp.src(demoFolder.path + '/**/*', { base: path.dirname(demoFolder.path) })
         .pipe(fileTasks(demoId))
         .pipe(through2.obj(function(file, enc, cb) {
           if (/index.html$/.test(file.path)) {