minor symfony#9947 Small grammar update in HttpFoundation (mattjanssen)

This PR was squashed before being merged into the 2.8 branch (closes symfony#9947).

Discussion
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Small grammar update in HttpFoundation

"... let you disable buffering at [the] config level or[by] adding ..."

Commits
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feb830f Small grammar update in HttpFoundation

Author: javiereguiluz

components/http_foundation.rst

@@ -473,7 +473,7 @@ represented by a PHP callable instead of a string::
 
     Additionally, PHP isn't the only layer that can buffer output. Your web
     server might also buffer based on its configuration. Some servers, such as
-    Nginx, let you disable buffering at config level or adding a special HTTP
+    Nginx, let you disable buffering at the config level or by adding a special HTTP
     header in the response::
 
         // disables FastCGI buffering in Nginx only for this response