Update WindowsBuild.md

Make the note on needing to adjust `PYTHON_EXECUTABLE` more prominent, and repeat in in the place where that argument actually needs to be adjusted.

Author: martinboehme

docs/WindowsBuild.md

@@ -21,7 +21,7 @@ In the Visual Studio installation program, under *Individual Components*
 
 1. Install *Python 2*, either the 32-bit version (C:\Python27\\) or the 64-bit version (C:\Python27amd64\\)
 
-> If you install the 64-bit version only, you will need to adjust `PYTHON_EXECUTABLE` below to `C:\Python27amd64\python.exe`
+   **Note:** If you install the 64-bit version only, you will need to adjust `PYTHON_EXECUTABLE` below to `C:\Python27amd64\python.exe`
 
 2. Install *Python 3 64 bits (3.7.x)*
 
@@ -139,6 +139,9 @@ cmake -B "S:\b\toolchain" ^
 ninja -C S:\b\toolchain
 ```
 
+**Note:** If you installed only the 64-bit version of Python, you will need to adjust `PYTHON_EXECUTABLE` argument to `C:\Python27amd64\python.exe`
+
+
 ## Running Swift tests on Windows
 
 ```cmd