[Grammar] Fix passive tense.

"be built".

---

I hereby disclaim any implicit or explicit ownership of my changes in this
changeset, and put them under a multiple licence consisting of your choice of
one of more of:

- The CC0 / Public Domain - https://creativecommons.org/choose/zero/ .

- The MIT / Expat license - https://en.wikipedia.org/wiki/MIT_License

- The default licence of your project

- The https://en.wikipedia.org/wiki/GNU_Lesser_General_Public_License - version
2.1 or higher

- The https://en.wikipedia.org/wiki/GNU_General_Public_License - version 2 or
higher

- Any licence in the 2018-Aug-27 popular licenses list of
https://opensource.org/licenses

- The https://en.wikipedia.org/wiki/Apache_License version 2.0 or later

- The https://en.wikipedia.org/wiki/Artistic_License version 2.0 or later

- The https://en.wikipedia.org/wiki/ISC_license

- The https://opensource.org/licenses/BSD-2-Clause

Crediting me will be nice, but not mandatory, and you can change the licence
of the project without needing my permission.

README.md

@@ -45,7 +45,7 @@ The resulting binaries will be found in `<path-to-marl>/build`
 
 ### Windows
 
-Marl can be build using [Visual Studio 2019's CMake integration](https://docs.microsoft.com/en-us/cpp/build/cmake-projects-in-visual-studio?view=vs-2019).
+Marl can be built using [Visual Studio 2019's CMake integration](https://docs.microsoft.com/en-us/cpp/build/cmake-projects-in-visual-studio?view=vs-2019).
 
 ### Using Marl in your CMake project