INTRO: Make headings bigger

Update heading levels bigger to match other headings on the web site.

Author: iriark01

docs/lib/INTRO.md

@@ -1,4 +1,4 @@
-### uVisor
+## uVisor
 
 The uVisor is a self-contained software hypervisor that creates independent secure domains on ARM Cortex®-M3 and Cortex®-M4 microcontrollers. It increases resilience against malware and protects secrets from leaking even among different modules of the same application. You can find a [high-level overview here](http://www.slideshare.net/FoolsDelight/practical-realtime-operating-system-security-for-the-masses) ([Download PDF](https://github.com/ARMmbed/uvisor/raw/docs/uVisorSecurity-TechCon2016.pdf)).
 
@@ -12,7 +12,7 @@ When the uVisor is enabled, all NVIC APIs are rerouted to the corresponding uVis
 
 Although this behavior is different from that of the original NVIC, it is backward compatible. Legacy code (such as a device HAL) still works after uVisor is enabled.
 
-#### Usage
+### Usage
 
 1. Include uVisor library in your application.
 1. Specify capabilities in mandatory access control list.
@@ -22,20 +22,20 @@ Although this behavior is different from that of the original NVIC, it is backwa
 
 For detailed usage guides and best practices, look [here](https://github.com/ARMmbed/uvisor/blob/master/docs/README.md)
 
-#### API
+### API
 
 [https://github.com/ARMmbed/uvisor/blob/master/docs/lib/API.md](https://github.com/ARMmbed/uvisor/blob/master/docs/lib/API.md)
 
-#### Example
+### Examples
 
 This is a simple example to show how to write a uVisor-secured threaded application with IRQ support. One LED blinks periodically from the public box main thread. A secure box exclusively owns the second LED, which toggles you press the user button.
 
-##### main.cpp
+#### main.cpp
 [![View code](https://www.mbed.com/embed/?url=https://github.com/ARMmbed/mbed-os-example-uvisor)](https://github.com/ARMmbed/mbed-os-example-uvisor/blob/master/source/main.cpp)
 
-##### main-hw.h
+#### main-hw.h
 [![View code](https://www.mbed.com/embed/?url=https://github.com/ARMmbed/mbed-os-example-uvisor)](https://github.com/ARMmbed/mbed-os-example-uvisor/blob/master/source/main-hw.h)
 
-##### led.cpp
+#### led.cpp
 
 [![View code](https://www.mbed.com/embed/?url=https://github.com/ARMmbed/mbed-os-example-uvisor)](https://github.com/ARMmbed/mbed-os-example-uvisor/blob/master/source/led.cpp)