docs: fix some typos in comments and strings

Couple of typos fixed:
- occuring -> occurring
- imlement -> implement
- idenitifer -> identifer
etc...

Closes #5943

modules/angular2/src/common/forms/model.ts

@@ -14,7 +14,7 @@ export const VALID = "VALID";
 export const INVALID = "INVALID";
 
 /**
- * Indicates that a Control is pending, i.e. that async validation is occuring and
+ * Indicates that a Control is pending, i.e. that async validation is occurring and
  * errors are not yet available for the input value.
  */
 export const PENDING = "PENDING";