Bump version in Cargo.toml

v0.5.2 was released on December 15, but I did something wrong when doing
the release/push cycle which made the Cargo.toml change not appear in
master.

The tag `v0.5.2` and changelog point to the actual released version of
v0.5.2

Author: TheZoq2

Cargo.toml

@@ -9,7 +9,7 @@ repository = "https://github.com/stm32-rs/stm32f1xx-hal"
 documentation = "https://docs.rs/stm32f1xx-hal"
 readme = "README.md"
 edition = "2018"
-version = "0.5.1"
+version = "0.5.2"
 
 [package.metadata.docs.rs]
 features = ["stm32f103", "rt", "stm32-usbd"]