Updates the version number in CMakeLists.txt to 1.12.1 (#3919)

Due to an oversight version 1.12.0 didn't update the version in
CMakeLists.txt. A patch release will fix this issue.

I've also added a note to instructions for creating a release to check
for this issue.

CMakeLists.txt

@@ -12,7 +12,7 @@ if (POLICY CMP0077)
 endif (POLICY CMP0077)
 
 project(googletest-distribution)
-set(GOOGLETEST_VERSION 1.11.0)
+set(GOOGLETEST_VERSION 1.12.1)
 
 if(NOT CYGWIN AND NOT MSYS AND NOT ${CMAKE_SYSTEM_NAME} STREQUAL QNX)
   set(CMAKE_CXX_EXTENSIONS OFF)