use const instead of let for SDK as it doesn't change

Author: kirkins

src/data/docs/slp/js/getting-started.md

@@ -26,5 +26,5 @@ To use inside a command line script or client/server app:
 
 Use the [scaffold generator](./new), or manually import SLP SDK into your code:
 
-    let SLPSDK = require('slp-sdk');
-    let SLP = new SLPSDK();
+    const SLPSDK = require('slp-sdk');
+    const SLP = new SLPSDK();

src/data/docs/slp/js/slp.md

@@ -24,7 +24,7 @@ SLP `SLP`: instance of SLP SDK
 
 ##### Create instance of SLP SDK
 
-    let SLPSDK = require("slp-sdk");
-    let SLP = new SLPSDK({
+    const SLPSDK = require("slp-sdk");
+    const SLP = new SLPSDK({
       restURL: "https://trest.bitcoin.com/v2/"
     });

src/data/docs/slp/js/slpdb.md

@@ -20,8 +20,8 @@ result `any`: SLPDB Result
 
 ```javascript
 ;(async () => {
-  let SLPSDK = require('slp-sdk')
-  let SLP = new SLPSDK()
+  const SLPSDK = require('slp-sdk')
+  const SLP = new SLPSDK()
   let res = await SLP.SLPDB.get({
     v: 3,
     q: {