MINOR: update install instruction (#400)

We have frequent releases and honoring semver, removed minor and patch version pinning
diff --git a/parquet/README.md b/parquet/README.md
index 326c966..7f47b56 100644
--- a/parquet/README.md
+++ b/parquet/README.md
@@ -27,7 +27,7 @@
 
 ```toml
 [dependencies]
-parquet = "4.1.0"
+parquet = "^4"
 ```
 
 and this to your crate root: