Remove extra space in further_reading.md

Remove extra space.

docs/tutorials/quickstart/further_reading.md

@@ -3,15 +3,15 @@
 - [Mbed OS tutorials and examples](../tutorials/index.html): list of Mbed OS 5 example repositories.
 
 - Documentation:
-    - [Mbed OS APIs](../apis/index.html): list of all APIs available in Mbed OS.
-    - [Peripheral drivers](../apis/drivers.html): I/O driver APIs (I2C, SPI, UART and so on).
+   - [Mbed OS APIs](../apis/index.html): list of all APIs available in Mbed OS.
+   - [Peripheral drivers](../apis/drivers.html): I/O driver APIs (I2C, SPI, UART and so on).
 
 - Tutorials:
-    - [Advanced debugging](../tutorials/debugging.html).
-    - [Serial communications](../tutorials/serial-communication.html).
-    - [Optimizing binary size](../tutorials/optimizing.html).
+   - [Advanced debugging](../tutorials/debugging.html).
+   - [Serial communications](../tutorials/serial-communication.html).
+   - [Optimizing binary size](../tutorials/optimizing.html).
 
 - Other resources:
-    - [Components database](https://os.mbed.com/components/): libraries and example code for various hardware and software components.
-    - [Mbed OS forum](https://os.mbed.com/forum/): a resource of questions and answers and an active user community. Ask your questions here first.
-    - [Youtube channel](http://youtube.com/armmbed): videos and workshop content.
+   - [Components database](https://os.mbed.com/components/): libraries and example code for various hardware and software components.
+   - [Mbed OS forum](https://os.mbed.com/forum/): a resource of questions and answers and an active user community. Ask your questions here first.
+   - [Youtube channel](http://youtube.com/armmbed): videos and workshop content.