Fix typos in platforms input documentation

These are some copy/paste errors caused by basing the `platform` input's documentation on the `libraries` input documentation.

Author: per1234

README.md

@@ -53,14 +53,14 @@ Keys:
 Keys:
 - `source-url` - URL to clone the repository from. It must start with `git://` or end with `.git`.
 - `version` - [Git ref](https://git-scm.com/book/en/v2/Git-Internals-Git-References) of the repository to checkout. The special version name `latest` will cause the latest tag to be used. By default, the repository will be checked out to the tip of the default branch.
-- `source-path` - path to install as a library. Paths are relative to the root of the repository. The default is to install from the root of the repository.
+- `source-path` - path to install as a platform. Paths are relative to the root of the repository. The default is to install from the root of the repository.
 - `name` - platform name in the form of `VENDOR:ARCHITECTURE`.
 
 ##### Archive download
 
 Keys:
 - `source-url` - download URL for the archive (e.g., `https://github.com/arduino/ArduinoCore-avr/archive/master.zip`).
-- `source-path` - path to install as a library. Paths are relative to the root folder of the archive, or the root of the archive if it has no root folder. The default is to install from the root folder of the archive.
+- `source-path` - path to install as a platform. Paths are relative to the root folder of the archive, or the root of the archive if it has no root folder. The default is to install from the root folder of the archive.
 - `name` - platform name in the form of `VENDOR:ARCHITECTURE`.
 
 ### `libraries`