fix(README): correct syntax errors

Remove `-` from variable names.

README.md

@@ -25,19 +25,19 @@ I reccomend using an optimized build matching your Node.js environment version,
  * Node 6
  * Built using `babel-preset-es2015-node6`
  */
-const mkdirp-promise = require('mkdirp-promise/lib/node6')
+const mkdirpPromise = require('mkdirp-promise/lib/node6')
 
 /*
  * Node 5
  * Built using `babel-preset-es2015-node5`
  */
-const mkdirp-promise = require('mkdirp-promise/lib/node5')
+const mkdirpPromise = require('mkdirp-promise/lib/node5')
 
 /*
  * Node 4
  * Built using `babel-preset-es2015-node4`
  */
-const mkdirp-promise = require('mkdirp-promise/lib/node4')
+const mkdirpPromise = require('mkdirp-promise/lib/node4')
 
 /*
  * Node >=0.10 <=0.12
@@ -46,13 +46,13 @@ const mkdirp-promise = require('mkdirp-promise/lib/node4')
  *   - additional package is required: `babel-runtime`
  *   - npm install --production --save babel-runtime
  */
-var mkdirp-promise = require('mkdirp-promise')
+var mkdirpPromise = require('mkdirp-promise')
 ```
 
 ## API
 
 ```js
-const mkdirp = require('mkdirp-promise')
+const mkdirpPromise = require('mkdirp-promise')
 ```
 
 ### mkdirp(dir, [, options])