Use standardized term "Boards Manager" in readme

Arduino CLI diverged from the other official Arduino development tools by calling it "Board Manager" rather that "Boards Manager", but it has been determined that the standardized term "Boards Manager" will be used everywhere.

Author: per1234

README.md

@@ -20,13 +20,13 @@ For 3rd party boards, also specify the Boards Manager URL:
 
 YAML-format list of platform dependencies to install.
 
-Default `""`. If no `platforms` input is provided, the board's dependency will be automatically determined from the `fqbn` input and the latest version of that platform will be installed via Board Manager.
+Default `""`. If no `platforms` input is provided, the board's dependency will be automatically determined from the `fqbn` input and the latest version of that platform will be installed via Boards Manager.
 
-If a platform dependency from a non-Board Manager source of the same name as another Board Manager source platform dependency is defined, they will both be installed, with the non-Board Manager dependency overwriting the Board Manager platform installation. This permits testing against a non-release version of a platform while using Board Manager to install the platform's tools dependencies.
+If a platform dependency from a non-Boards Manager source of the same name as another Boards Manager source platform dependency is defined, they will both be installed, with the non-Boards Manager dependency overwriting the Boards Manager platform installation. This permits testing against a non-release version of a platform while using Boards Manager to install the platform's tools dependencies.
 Example:
 ```yaml
 platforms: |
-  # Install the latest release of Arduino SAMD Boards and its toolchain via Board Manager
+  # Install the latest release of Arduino SAMD Boards and its toolchain via Boards Manager
   - name: "arduino:samd"
   # Install the platform from the root of the repository, replacing the BM installed platform
   - source-path: "."
@@ -35,7 +35,7 @@ platforms: |
 
 #### Sources:
 
-##### Board Manager
+##### Boards Manager
 
 Keys:
 - `name` - platform name in the form of `VENDOR:ARCHITECTURE`.