fix a comment grammar-o

Author: nico

llvm/utils/gn/build/write_cmake_config.py

@@ -22,7 +22,7 @@
         #define FOO 0
 
 2.) #cmakedefine FOO [...]
-    Checks if key FOO is set to a truthy in value, and depending on that prints
+    Checks if key FOO is set to a truthy value, and depending on that prints
     one of the following two lines:
 
         #define FOO [...]