Update branch name in WINDOWS_PORT.md (facebook#8745)

Summary: Pull Request resolved: facebook#8745

Reviewed By: jay-zhuang

Differential Revision: D30718273

Pulled By: ltamasi

fbshipit-source-id: f0f4d592a71a327e731a5eef0f01488074d99118

Author: ltamasi

WINDOWS_PORT.md

@@ -24,7 +24,7 @@ We strive to achieve the following goals:
 * make all unit test pass both in debug and release builds. 
   * Note: latest introduction of SyncPoint seems to disable running db_test in Release.
 * make performance on par with published benchmarks accounting for HW differences
-* we would like to keep the port code inline with the master branch with no forking
+* we would like to keep the port code inline with the main branch with no forking
 
 ## Build system
 We have chosen CMake as a widely accepted build system to build the Windows port. It is very fast and convenient.