Edit guidelines.md

Fix existing typo, and spell out symbol for consistent style.

Author: Amanda Butler

docs/reference/contributing/guidelines/guidelines.md

@@ -2,11 +2,11 @@
 
 Consistency is an important aspect of a codebase. To ensure consistency in Mbed OS code, we have created contributing guidelines. Any contribution to Mbed OS needs to meet the following criteria:
 
-* Design and coding style: Be consistent with your changes. We define software design principles and coding style in [this document](../contributing/style.html).
-* Contributions guidance: The process and how we review contributions is described in the [workflow](../contributing/workflow.html) document.
-* Licenses: Licenses should comply with the [licenses described here](../contributing/license.html).
+- Design and coding style: Be consistent with your changes. We define software design principles and coding style in [this document](../contributing/style.html).
+- Contributions guidance: The process and how we review contributions is described in the [workflow](../contributing/workflow.html) document.
+- Licenses: Licenses should comply with the [licenses described here](../contributing/license.html).
 
-### Access to the ARMmbed organisation on GitHub
+### Access to the ARMmbed organization on GitHub
 
 You might require direct access to the ARMmbed organization for one of the following reasons:
 
@@ -26,4 +26,4 @@ Each repository must contain:
 
 - `CONTRIBUTING.md` file (similar to [Mbed OS contributing](https://github.com/ARMmbed/mbed-os/blob/master/CONTRIBUTING.md)).
 - `LICENSE` file - the full license text or overview of every license in the repo with links.
-- `README.md` has a license & contributing section (similar to [Mbed OS license and contributions section](https://github.com/ARMmbed/mbed-os/blob/master/README.md#license-and-contributions)).
+- `README.md` has a license and contributing section (similar to [Mbed OS license and contributions section](https://github.com/ARMmbed/mbed-os/blob/master/README.md#license-and-contributions)).