Fix documentation link version in README (#473)

Author: oameye

README.md

@@ -1,6 +1,6 @@
 <img src="./logo.png" width="750" align="top">
 
-[![docs](https://img.shields.io/badge/docs-online-blue.svg)](https://quantumengineeredsystems.github.io/HarmonicBalance.jl/V0.15)
+[![docs](https://img.shields.io/badge/docs-online-blue.svg)](https://quantumengineeredsystems.github.io/HarmonicBalance.jl/v0.15)
 [![Downloads](https://img.shields.io/badge/dynamic/json?url=http%3A%2F%2Fjuliapkgstats.com%2Fapi%2Fv1%2Ftotal_downloads%2FHarmonicBalance&query=total_requests&label=Downloads)](https://juliapkgstats.com/pkg/HarmonicBalance)
 [![Downloads](https://img.shields.io/badge/dynamic/json?url=http%3A%2F%2Fjuliapkgstats.com%2Fapi%2Fv1%2Fmonthly_downloads%2FHarmonicBalance&query=total_requests&suffix=%2Fmonth&label=Downloads)](https://juliapkgstats.com/pkg/HarmonicBalance)