minor symfony#5518 Minor grammar fix. (maxolasersquad)

This PR was submitted for the 2.7 branch but it was merged into the 2.3 branch instead (closes symfony#5518).

Discussion
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Minor grammar fix.

Commits
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e603430 Minor grammar fix.

Author: xabbuh

book/installation.rst

@@ -127,7 +127,7 @@ based on `Composer`_.
 
 Composer is the dependency manager used by modern PHP applications and it can
 also be used to create new applications based on the Symfony Framework. If you
-don't have installed it globally, start by reading the next section.
+don't have it installed globally, start by reading the next section.
 
 Installing Composer Globally
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~