Update documentation-guidelines.md

Fixed typo on line 44 and added "Sub-section Headings..."

doc/process/documentation-guidelines.md

@@ -41,7 +41,7 @@ This procedure works on Linux servers running Python 2.6 and higher.
 
 ### Use headings  
 
-Headings in markdown provide anchors that can be used to easily reference sub-sections of long pieces of documentation. This is preferrable to just numbering sections without headings. 
+Headings in markdown provide anchors that can be used to easily reference sub-sections of long pieces of documentation. This is preferable to just numbering sections without headings. 
 
 ##### Correct: 
 
@@ -79,6 +79,23 @@ These headers are smaller and used to summarize sections. H3 can be considered e
 
 These conventions are new, so there's flexibility around them, when you're not sure, consider the convention here as default.
 
+### Sub-section headings should end with a colon
+
+For readability and clear layout, end a sub-section heading with a colon
+
+##### Correct: 
+
+---- 
+Service Based: 
+- [AWS Elastic Beanstalk Setup](https://github.com/mattermost/platform/blob/master/doc/install/Amazon-Elastic-Beanstalk.md)
+---- 
+##### Incorrect: 
+
+---- 
+Optional
+- [Community Guide for Production Debian Setup](https://github.com/mattermost/platform/blob/master/doc/install/Production-Debian.md)
+---- 
+
 ### One instruction per line
 
 It's easy to miss instructions when they're compounded. Have only one instruction per line, so documentation looks more like a checklist.