s/Python-dotenv/python-dotenv/ (#516)

This commit uses the name `python-dotenv` instead of `Python-dotenv` in the README.md file

Author: chapeupreto

README.md

@@ -3,7 +3,7 @@
 [![Build Status][build_status_badge]][build_status_link]
 [![PyPI version][pypi_badge]][pypi_link]
 
-Python-dotenv reads key-value pairs from a `.env` file and can set them as environment
+python-dotenv reads key-value pairs from a `.env` file and can set them as environment
 variables. It helps in the development of applications following the
 [12-factor](https://12factor.net/) principles.
 
@@ -29,7 +29,7 @@ If your application takes its configuration from environment variables, like a 1
 application, launching it in development is not very practical because you have to set
 those environment variables yourself.
 
-To help you with that, you can add Python-dotenv to your application to make it load the
+To help you with that, you can add python-dotenv to your application to make it load the
 configuration from a `.env` file when it is present (e.g. in development) while remaining
 configurable via the environment:
 
@@ -201,7 +201,7 @@ empty string.
 
 ### Variable expansion
 
-Python-dotenv can interpolate variables using POSIX variable expansion.
+python-dotenv can interpolate variables using POSIX variable expansion.
 
 With `load_dotenv(override=True)` or `dotenv_values()`, the value of a variable is the
 first of the values defined in the following list: