Docs: Change rule descriptions for consistent casing (#6915)

Scrolling through eslint.org/docs/rules, I found two descriptions that
began with uppercase letters, while all the rest were lowercase. For the
curious, here's how I ran a quick check to be sure I didn't miss any:

```js
Array.from(document.querySelectorAll('tr td:last-child p'))
    .map(p => p.innerText)
    .filter(description =>
        description[0].toLowerCase() !== description[0]
    )
```

lib/rules/no-tabs.js

@@ -17,7 +17,7 @@ const regex = /\t/;
 module.exports = {
     meta: {
         docs: {
-            description: "Disallow tabs in file",
+            description: "disallow tabs in file",
             category: "Stylistic Issues",
             recommended: false
         },

lib/rules/no-template-curly-in-string.js

@@ -11,7 +11,7 @@
 module.exports = {
     meta: {
         docs: {
-            description: "Disallow template literal placeholder syntax in regular strings",
+            description: "disallow template literal placeholder syntax in regular strings",
             category: "Possible Errors",
             recommended: false
         },