docs(en): fix capital letter (#1223)

I am sorry for that :)

Author: bichikim

docs/en/api.md

@@ -202,7 +202,7 @@ const store = new Vuex.Store({ ...options })
 
   The first argument can optionally be a namespace string. [Details](modules.md#binding-helpers-with-namespace)
 
-  The Second object argument's members can be a function. `function(state: any)` 
+  The second object argument's members can be a function. `function(state: any)` 
 
 - **`mapGetters(namespace?: string, map: Array<string> | Object<string>): Object`**
 
@@ -216,15 +216,15 @@ const store = new Vuex.Store({ ...options })
 
   The first argument can optionally be a namespace string. [Details](modules.md#binding-helpers-with-namespace)
 
-  The Second object argument's members can be a function. `function(dispatch: function, ...args: any[])`
+  The second object argument's members can be a function. `function(dispatch: function, ...args: any[])`
 
 - **`mapMutations(namespace?: string, map: Array<string> | Object<string | function>): Object`**
 
   Create component methods options that commit a mutation. [Details](mutations.md#committing-mutations-in-components)
 
   The first argument can optionally be a namespace string. [Details](modules.md#binding-helpers-with-namespace)
 
-  The Second object argument's members can be a function. `function(commit: function, ...args: any[])`
+  The second object argument's members can be a function. `function(commit: function, ...args: any[])`
 
 - **`createNamespacedHelpers(namespace: string): Object`**