fix(index): spelling and grammar in error messages

index.js

@@ -230,7 +230,7 @@ class RefResolver {
       if (sourceSchema[key] !== undefined) {
         if (deepEqual(sourceSchema[key], targetSchema[key])) continue
         throw new Error(
-          `Cannot resolve ref "${ref.ref}". Property "${key}" is already exist in schema "${ref.sourceSchemaId}".`
+          `Cannot resolve ref "${ref.ref}". Property "${key}" already exists in schema "${ref.sourceSchemaId}".`
         )
       }
       sourceSchema[key] = targetSchema[key]
@@ -250,7 +250,7 @@ class RefResolver {
   #insertSchemaByAnchor (schema, schemaId, anchor) {
     const { anchors } = this.#schemas[schemaId]
     if (anchors[anchor] !== undefined) {
-      throw new Error(`There is already another anchor "${anchor}" in a schema "${schemaId}".`)
+      throw new Error(`There is already another anchor "${anchor}" in schema "${schemaId}".`)
     }
     anchors[anchor] = schema
   }

test/collisions.test.js

@@ -194,6 +194,6 @@ test('should throw if there is the same anchor in the same schema', () => {
     refResolver.addSchema(schema)
     assert.fail('should throw')
   } catch (err) {
-    assert.equal(err.message, 'There is already another anchor "#subSchemaId" in a schema "schemaId1".')
+    assert.equal(err.message, 'There is already another anchor "#subSchemaId" in schema "schemaId1".')
   }
 })

test/deref-schema.test.js

@@ -45,7 +45,7 @@ test('should throw id source schema has a key with a same key as ref schema, but
   } catch (err) {
     assert.strictEqual(
       err.message,
-      'Cannot resolve ref "schemaId2". Property "properties" is already exist in schema "schemaId1".'
+      'Cannot resolve ref "schemaId2". Property "properties" already exists in schema "schemaId1".'
     )
   }
 })