docs: more insightful leadin for Nx doc (#10017)

### Description

@JamesHenry pointed out that the "Migrating from Nx" doc leaves room to
make it sound like the "Why switch?" reasons sound like they come from
the Turborepo team. Here, making sure that it's known on the page that
that's not the case.

Thanks, James!

Author: anthonyshew

docs/repo-docs/guides/migrating-from-nx.mdx

@@ -12,7 +12,7 @@ This guide will help you migrate an existing Nx repository to Turborepo.
 
 ## Why switch?
 
-There are several reasons why you might want to switch from from Nx to Turborepo.
+There are many reasons why you may be choosing to migrate from Nx to Turborepo. Below, we've listed the most common reasons that developers have referenced for their migrations.
 
 ### Ecosystem standards