Fixed typos on lines 133 and 157 (#5409)

I also changed the wording on line 169 to be a bit more intuitive

lib/mixins/element-mixin.js

@@ -130,7 +130,7 @@ export const ElementMixin = dedupingMixin(base => {
   }
 
   /**
-   * Returns a memoized version of the the `observers` array.
+   * Returns a memoized version of the `observers` array.
    * @param {PolymerElementConstructor} constructor Element class
    * @return {Array} Array containing own observers for the given class
    * @protected
@@ -154,7 +154,7 @@ export const ElementMixin = dedupingMixin(base => {
    * alter these settings. However, additional `observers` may be added
    * by subclasses.
    *
-   * The info object should may contain property metadata as follows:
+   * The info object should contain property metadata as follows:
    *
    * * `type`: {function} type to which an attribute matching the property
    * is deserialized. Note the property is camel-cased from a dash-cased
@@ -166,7 +166,7 @@ export const ElementMixin = dedupingMixin(base => {
    * property 'foo',
    *
    * * `computed`: {string} creates a computed property. A computed property
-   * also automatically is set to `readOnly: true`. The value is calculated
+   * is also automatically set to `readOnly: true`. The value is calculated
    * by running a method and arguments parsed from the given string. For
    * example 'compute(foo)' will compute a given property when the
    * 'foo' property changes by executing the 'compute' method. This method