Change docs links to actual file extension (.md)

See https://gerrit-review.googlesource.com/83950 for detailed context.

Change-Id: I425285fc81404d49629bf2f468f45c3fc67dd8f9

Author: Yuxuan 'fishy' Wang

src/main/resources/Documentation/about.md

@@ -7,7 +7,7 @@ Download Schemes
 
 The following download schemes are defined by this plugin. Please note
 that some download schemes must be enabled by [configuration]
-(config.html#download.scheme).
+(config.md#download.scheme).
 
 * `Anonymous Git`: Scheme for anonymous downloads via the Git protocol.
 
@@ -26,7 +26,7 @@ Download Commands
 
 The following download commands are defined by this plugin. Please note
 that some download commands must be enabled by [configuration]
-(config.html#download.command).
+(config.md#download.command).
 
 ### Git Commands