Correct the description of _.isString

The previous description was copy-pasted from _.endsWith. This updated description comes from https://lodash.com/docs/4.17.15#isString.

README.md

@@ -2437,7 +2437,7 @@ Checks if string ends with the given target string.
 
 ### _.isString
 :heavy_exclamation_mark:`Not in Underscore.js`
-Checks if string ends with the given target string.
+Checks if value is classified as a String primitive or object.
 
   ```js
   // Lodash