chore: add missing quotes in README.md (#656)

We don't have `2.1.5` in js, but `'2.1.5'`

Author: zyxkad

README.md

@@ -543,7 +543,7 @@ ex.
 * `s.clean(' = v 2.1.5-foo')`: `null`
 * `s.clean(' = v 2.1.5-foo', { loose: true })`: `'2.1.5-foo'`
 * `s.clean('=v2.1.5')`: `'2.1.5'`
-* `s.clean('  =v2.1.5')`: `2.1.5`
+* `s.clean('  =v2.1.5')`: `'2.1.5'`
 * `s.clean('      2.1.5   ')`: `'2.1.5'`
 * `s.clean('~1.0.0')`: `null`