correct JTD discriminator docs

docs/json-type-definition.md

@@ -170,11 +170,11 @@ Invalid data: `{}`, `{foo: 1}`, `{foo: "bar", bar: "3"}`, any type other than ob
 
 This form defines discriminated (tagged) union of different record types.
 
-It has required members `discriminator` and `mappings` and optional members `nullable` and `metadata`, no other members are allowed.
+It has required members `discriminator` and `mapping` and optional members `nullable` and `metadata`, no other members are allowed.
 
-The string value of `discriminator` schema member contains the name of the data member that is the tag of the union. `mappings` schema member contains the dictionary of schemas that are applied according to the value of the tag member in the data. Schemas inside `mappings` must have "properties" form.
+The string value of `discriminator` schema member contains the name of the data member that is the tag of the union. `mapping` schema member contains the dictionary of schemas that are applied according to the value of the tag member in the data. Schemas inside `mapping` must have "properties" form.
 
-Properties forms inside `mappings` cannot be `nullable` and cannot define the same property as discriminator tag.
+Properties forms inside `mapping` cannot be `nullable` and cannot define the same property as discriminator tag.
 
 **Example 1.**
 
@@ -183,7 +183,7 @@ Schema:
 ```javascript
 {
   discriminator: "version",
-  mappings: {
+  mapping: {
     "1": {
       properties: {
         foo: {type: "string"}
@@ -202,12 +202,12 @@ Valid data: `{version: "1", foo: "1"}`, `{version: "2", foo: 1}`
 
 Invalid data: `{}`, `{foo: "1"}`, `{version: 1, foo: "1"}`, any type other than object
 
-**Example 3: invalid schema (discriminator tag member defined in mappings)**
+**Example 3: invalid schema (discriminator tag member defined in mapping)**
 
 ```javascript
 {
   discriminator: "version",
-  mappings: {
+  mapping: {
     "1": {
       properties: {
         version: {enum: ["1"]},