fix(docs) use let instead of const for variable which gets changed

`const` variables cannot be reassigned.

docs/_templates/page.html

@@ -29,7 +29,7 @@ function add_version_selector()
             p.innerHTML = `
             <select name="versions" id="versions" onchange="ver_sel()" style="border-radius:5px; margin-bottom:15px">
             ${versions.map(version => {
-                const versionName = "";  
+                let versionName = "";  
                 if(version == "master") versionName = "master (latest)";
                 else versionName = "v" + ((version.indexOf(".") != -1) ? version : (version + " (latest minor)"));
                 return `<option value="${version}">${versionName}</option>`;