Replace var with const in README.md

README code style should satisfy the ESLint rules.

Signed-off-by: Kevin Locke <kevin@kevinlocke.name>

Author: kevinoid

README.md

@@ -112,7 +112,7 @@ $ echo $?
 To use the API with a callback function:
 
 ```js
-var gitBranchIs = require('git-branch-is');
+const gitBranchIs = require('git-branch-is');
 gitBranchIs('main', function(err, result) {
   if (err) console.error(err);
   else console.log(result ? 'On main' : 'Not on main');
@@ -123,7 +123,7 @@ Alternatively, if a callback is not provided, `gitBranchIs` will return a
 `Promise`:
 
 ```js
-var gitBranchIs = require('git-branch-is');
+const gitBranchIs = require('git-branch-is');
 gitBranchIs('main').then(
   function(result) { console.log(result ? 'On main' : 'Not on main'); },
   function(err) { console.error(err); }
@@ -134,7 +134,7 @@ Additionally, instead of a string, a checking function can be passed to
 perform arbitrary checking against the branch name:
 
 ```js
-var gitBranchIs = require('git-branch-is');
+const gitBranchIs = require('git-branch-is');
 gitBranchIs(function(branchName) { /^main$/.test(branchName); }).then(
   function(result) { console.log(result ? 'On main' : 'Not on main'); },
   function(err) { console.error(err); }