Update index.md

Continuing to clean-up typos.

doc/sphinx/user_manual/introduction/index.md

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 Introduction
 =============
 
-Welcome to the user manual of the Geodynamic World Builder, or in short GWB! This manual part contain all the information you need to get started with building your geodynamic models. 
+Welcome to the User Manual of the Geodynamic World Builder, or in short GWB! The User Manual contains all the information you need to get started with building your geodynamic models. 
 
-This introduction chapter will contain some basic information you should know about the GWB before you get started. The next chapter will go trough the installation instructions and explain how to use the different apps or tool. Next we will introduce you to some important concepts to understand how the GWB allows you to easily build complex parameterized initial conditions for geodynamic models or geodynamic visualization. The basic Starter Tutorial is the place where you will get to build a 3D (and 2D) subduction setup from scratch. This tutorial will take you step-by-step through the process and will explain how everything connects. Lastly, there is a cookbook chapter which contains some more stand-alone advanced topics.
+This introduction chapter will contain some basic information you should know about the GWB before you get started. The next chapter will go through the installation instructions and explain how to use the different apps or tools. Next, we will introduce you to some important concepts to understand how the GWB allows you to easily build complex, parameterized initial conditions for geodynamic models or geodynamic visualization. The basic Starter Tutorial is the place where you will get to build a 3D (and 2D) subduction setup from scratch. This tutorial will take you step-by-step through the process and will explain how everything connects. Lastly, there is a cookbook chapter which contains some more stand-alone advanced topics.
 
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