Rename prevSiblingsIterator to previousSiblingsIterator

This is consistent with `previousSibling`

Author: Joris-van-der-Wel

lib/SymbolTree.js

@@ -378,12 +378,12 @@ class SymbolTree {
          *
          * `O(1)` for a single iteration
          *
-         * @method prevSiblingsIterator
+         * @method previousSiblingsIterator
          * @memberOf module:symbol-tree#
          * @param {Object} object
          * @return {Object} An iterable iterator (ES6)
          */
-        prevSiblingsIterator(object) {
+        previousSiblingsIterator(object) {
                 return new TreeIterator(
                         this,
                         object,

test/SymbolTree.js

@@ -771,7 +771,7 @@ test('previous sibling iterator', function(t) {
 
         const results = [];
 
-        for (const object of tree.prevSiblingsIterator(ad)) {
+        for (const object of tree.previousSiblingsIterator(ad)) {
                 results.push(object);
         }
         t.deepEqual([ac, ab, aa], results);